At the Portsmouth commemorations - hosted by Dame Helen Mirren - William read an extract from the diary of Captain Alastair Bannerman, a soldier who was part of D-Day, addressed to his wife on the morning of the landings.
He told the flag-waving crowd he was 'deeply honoured' to be part of proceedings on Southsea Common and said 'we will always remember those who served'.
Delivering his speech to the crowd, the King said: 'The stories of courage, resilience and solidarity which you have heard today and throughout our lives cannot fail to move us, to inspire us and to remind us of what we owe to that great wartime generation, now tragically dwindling to so few.
'It is our privilege to hear that testimony, but our role is not purely passive. It is our duty to ensure that we and future generations do not forget their service and their sacrifice in replacing tyranny with freedom.'
Charles faced breezy conditions on stage as he spoke, with his speech notes blowing around in his hand.
In his programme notes for the event, the King spoke of his 'profound admiration and respect' for those who took part in D-Day, adding: 'It remains our solemn duty to continue to honour the outstanding gallantry, service and sacrifice of those who took part in that perilous mission.'
D-Day veterans, armed forces personnel and local schoolchildren were among the guests at the star-studded event held on Southsea Common in Portsmouth.
The Hampshire port city was a major staging post for the key Second World War battle, having been the base for the vessels and troops heading to Sword Beach, and nearby Southwick was the headquarters for Supreme Allied Commander General Dwight Eisenhower, who led the Operation Overlord planning.
The commemoration, which took place on a specially erected stage, was kicked off with a flypast by Dakota aircraft before host Dame Helen Mirren took to the stage to narrate a history of how D-Day unfolded and affected those who took part.
The event featured veterans reading dramatic accounts from the invasion as well as wartime songs performed by EastEnders star Emma Barton, American actress Marisha Wallace and Call The Midwife's Helen George.
What role did his ship HMS Erebus play in the Normandy landings?
D-Day veteran Eric Bateman served on board HMS Erebus, a Royal Navy warship.
Launched in 1915, HMS Erebus entered active service a year later.
One of its first major missions was a bombardment of Belgian ports where enemy warships were based, which led to the ship being seriously damaged.
It was later refitted and entered active service again following the outbreak of the Second World War.
During the Normandy landings of June 6, 1944 it accompanied American troops as they landed on Sword Beach - bombarding the German batteries of Barfleur and La Pernelle.
In the summer of 1944 it made its way to Le Havre to support invading Allied forces, before performing another tour of duty in Holland.
HMS Erebus was retired after the end of WWII and scrapped in July 1946.
Music was also provided by the Royal Marines Drummers.
The King and Queen took to the stage for Charles to give a reading followed later in proceedings by Rishi Sunak.
Readings were also given by Jonny Weldon from The Outlaws and Kate Phillips from Peaky Blinders as well as Iain Glen from Game Of Thrones, Leonie Elliott, also from Call The Midwife, Anjli Mohindra from Bodyguard and Julian Ovenden from Downton Abbey.
The show concluded with a gun salute from the frigate HMS St Albans, which sailed past in the Solent, and a rendition of the national anthem before a flypast by the Red Arrows and Typhoon jets.
Mr Sunak read an address by Field Marshal Montgomery, which was delivered to the troops ahead of the D-Day landings.
The Prime Minister read out the short address which began: 'The time has come to deal the enemy a terrific blow in Western Europe. The blow will be struck by the combined sea, land and air forces of the Allies together constituting one great allied team, under the supreme command of General Eisenhower.'
It continued: 'To us is given the honour of striking a blow for freedom which will live in history; and in the better days that lie ahead men will speak with pride of our doings. We have a great and a righteous cause.'
Prince William took centre stage at the event by making the opening address, in which he spoke of the families who 'watched their loved ones go into battle' as they left for the largest seaborne invasion in history in June 1944.
Addressing the crowd at Southsea Common, William said: 'We will always remember those who served and those who waved them off.
'The mothers and fathers, brothers and sisters, sons and daughters who watched their loved ones go into battle, unsure if they would ever return.
'Today we remember the bravery of those who crossed this sea to liberate Europe. Those who ensured that Operation Overlord was a success. And those who waited for their safe return.'
He said those who fought on D-Day 'came from across our nation and from all walks of life to join in the fight against tyranny'.
'Many of those that took up arms had never seen combat before, some were still only in their teens,' he added.
The prince read a moving and poetic letter by Captain Alastair Bannerman of the Royal Warwickshire Regiment, written in his diary at 3am on the morning of D-Day, just hours before the landings, and addressed to his wife.
It spoke of the moon shining though heavy clouds and the 'row of small ships and of darker balloons silhouetted in front and behind us against the grey sea'.
Captain Bannerman thought of his wife and two young sons asleep in their nursery - and photographs of the soldier and his blond-haired children were shown on the screen behind William as he read the words.
''I can imagine how you listen to the news at 9 o'clock and think of me with love. I hope that Andrew's golden head rests gently and quietly upon his small pillow and that Richard is nice and comfortable lying in his narrow little carry-cot',' the prince said.
Charles and Camilla on stage at the commemorative event on Southsea Common in Portsmouth
The Prince of Wales shakes hands with Mr Bateman after he delivered his address
Camilla - who had been visibly moved by Mr Bateman's speech - greets him today
Members of the public watch as the Red Arrows perform a flypast above HMS St Albans, a Type 23 Frigate
The King smiles as he walks onto the stage alongside Queen Camilla
Charles acknowledges a standing ovation from the crowd as he walks onto the stage today
The King wore military medals for the major event in Portsmouth
The King, who was giving his first public speech since being diagnosed with cancer, received a standing ovation
After his speech, Charles took a seat in the Royal Box alongside William and Camilla
Prime Minister Rishi Sunak also gave an address at the event
Dame Helen, 78, beckons to the crowd assembled on Southsea Common in Portsmouth
The actress praised the assembled veterans for their bravery, saying: 'The presence today of some of those who contributed to that remarkable venture is an extraordinary privilege'
A member of the audience watches on during today's event in Portsmouth
Singer Marisha Wallace performs 'Sing, Sing, Sing' during the UK's national commemorative event
The performance by Marisha Wallace was witnessed by a large crowd of veterans and wellwishers
Leonie Elliott, best known for her role as Lucille Anderson in the BBC series Call the Midwife, speaks at the event
Service personnel watching on during the commemorative event for the 80th anniversary of D-Day
Artists perform on stage during the event on Southsea Common in Portsmouth
Prime Minister Rishi Sunak meets Mr Bateman, who delivered a moving address
Earlier, William praised the 'bravery' of those who took part in the D-Day landings and read an extract from a letter by Captain Alastair Bannerman of the Royal Warwickshire Regiment
William continued: ''The whole Channel between us and Cherbourg is filled with little ships which all quietly and efficiently sail towards France.
Prince William's reading at the D-Day event in Portsmouth
I am deeply honoured to join you today to recognise the bravery of all of those like John Haddock who participated in the D-Day landings, the start of the liberation of France and Europe, that led to victory of the Allied Powers.
Our guests of honour today are those veterans from the Normandy landing, that came from across our nation and from all walks of life to join in the fight against tyranny. Many of those that took up arms had never seen combat before, some were still only in their teens.
In their letters and diaries we see their great strength.
This is an extract from a letter by Captain Alastair Bannerman of the Royal Warwickshire Regiment, addressed to his wife and written in his diary on the morning of D-Day:
It is now 03.00 hours in the morning and I have just been up to the bridge. It is rather light because the moon is shining, though heavy clouds cover her. One can see the row of small ships and of darker balloons silhouetted in front and behind us against the grey sea.
You, my angel, sleep gently in the nursery, I hope. Your thoughts have helped me so much. They have given me real strength. I can imagine how you listen to the news at 9 o'clock and think of me with love. I hope that Andrew's golden head rests gently and quietly upon his small pillow and that Richard is nice and comfortable lying in his narrow little carry-cot.
A long line of flares hangs over Cherbourg, or I suppose it is Cherbourg, and a few anti-aircraft tracer shots go up in the air above the immediate front line. Funny to imagine that there Germans run around their guns. I would like to know what they are thinking. The whole Channel between us and Cherbourg is filled with little ships which all quietly and efficiently sail towards France. The British, Canadian and American fighting forces on the war-path.
I do not believe that I can now write for very long. We can now see the French coast and very soon we will have to play our part.
I must go now and look for the landing markings with my binoculars to ascertain our landing points. So, my darling, on we go! I know that you are with me. Au revoir, God bless, I love you!
Captain Bannerman survived the landings and the war, being taken prisoner and returning home to his wife and sons in April the next year. Too many never returned. They remain in 'some corner of a foreign field that is forever England.'
We will always remember those who served and those who waved them off. The mothers and fathers, brothers and sisters, sons and daughters who watched their loved ones go into battle, unsure if they would ever return.
Today we remember the bravery of those who crossed this sea to liberate Europe. Those who ensured that Operation Overlord was a success. And those who waited for their safe return.
''The British, Canadian and American fighting forces on the war-path.
''I do not believe that I can now write for very long. We can now see the French coast and very soon we will have to play our part.
''I must go now and look for the landing markings with my binoculars to ascertain our landing points. So, my darling, on we go! I know that you are with me. Au revoir, God bless, I love you!''
William shared how Captain Bannerman survived the landings and the war, was taken prisoner, but returned home to his wife and sons in April the next year.
He added, quoting Rupert Brooke's famous war poem The Soldier: 'Too many never returned. They remain in 'some corner of a foreign field that is forever England'.'
The prince said he was 'deeply honoured' to join the commemoration to recognise the bravery of those who took part in the landings.
William delivered his speech before the arrival of the King and Queen, with Charles also making his own address.
During the service, he sat between Prime Minister Rishi Sunak and Defence Secretary Grant Shapps.
The prince was wearing his Golden Jubilee, Diamond Jubilee, Platinum Jubilee and Coronation medals and his Great Master of The Most Honourable Order of the Bath neck order.
Dame Helen Mirren formerly introduced the event just after 11am by addressing the assembled veterans, saying: 'The presence today of some of those who contributed to that remarkable venture is an extraordinary privilege.
'Your bravery remains as inspiring now as it was eight decades ago. Thank you.'
The commemorations in Portsmouth opened with a musical performance and a flypast of two historic Dakota military transport aircraft, widely used by the Allies during the Second World War.
Charles and Queen Camilla will join the Ministry of Defence and Royal British Legion's commemorative event at the British Normandy Memorial at Ver-sur-Mer tomorrow.
Charles is said to have been determined to mark the landmark anniversary given it will be the last event of its kind that D-Day veterans will be able to witness.
This has been aided by a good recovery that has allowed his public duties to be brought forward by 'about a month', a source told The Times.
Meanwhile, William will attend the Canadian ceremony at Juno Beach Centre at Courseulles-sur-Mer hosted by the Canadian government, alongside veterans and Canadian armed forces personnel.
The Prince of Wales will then attend the international commemorative ceremony at Omaha Beach, Saint Laurent sur Mer, later on June 6 alongside 25 heads of state and veterans from around the world.
William took his place in the royal box today alongside military veterans shortly after Prime Minister Rishi Sunak and Labour leader Sir Keir Starmer.
Some veterans will attend two days of remembrance events in Portsmouth to mark the historic milestone.
FRANCE: D-Day veteran Henry Rice, 98, sheds a tear at the statue of Field Marshal Montgomery during the Spirit of Normandy Trust service in Coleville-Montgomery, France
French President Emmanuel Macron (R) speaks with Achille Muller (L), 98, last survivor of the Free French Forces, as they attend a ceremony commemorating SAS paratroopers and Free French Forces who died in Brittany during World War II, at La Gree mill SAS (Special Air Service) memorial in Plumelec, western France
France's President Emmanuel Macron (center R) and Achille Muller (center L), 98, last survivor of the Free French Forces, look on as a wreath is layed during a ceremony commemorating SAS paratroopers and Free French Forces who died
Princess Royal Anne attends a ceremony of unveiling of a statue of a Second World War Canadian Royal Regina Rifleman during a reception with members of the regiment to mark the 80th anniversary of D-Day, at Place des Canadiens in Bretteville-l'Orgueilleuse, Normandy
Members of the royal box stand and applaud at the start of this morning's ceremony
Prince William talks with Rishi Sunak at today's national commemorative event for the 80th anniversary of D-Day
William, wearing his military medals, alongside Mr Sunak and Ms Murty in Portsmouth today
William smiles as he greets David Cameron - who was among the dignitaries in Portsmouth
The Prince of Wales shakes hands with Defence Secretary Grant Shapps
The Normandy landings were the largest seaborne invasion in history, with the 1944 battle laying the foundation for an Allied victory.
Troops from the UK, the USA, Canada, and France attacked German forces on the beaches at Normandy in northern France on June 6, 1944.
Allied troops departed from Portsmouth on June 5, so the UK and French governments will host memorial events at both ports to commemorate the D-Day landings.
Over the course of two days, the King, Prime Minister Rishi Sunak, a slew of other dignitaries, and-most importantly-those who fought on the beaches in 1944 will come together to remember the battle that became the turning point towards an Allied victory.
However, the proceedings have already begun for dozens of other Second World War veterans.
Yesterday, a slew of former D-Day soldiers boarded a ferry to make the journey from Portsmouth to Normandy once again, retracing their steps 80 years ago.
The morning will culminate in a flypast by the Royal Air Force Red Arrows.
The Prime Minister will then deliver a reading to the crowds and meet with veterans to hear their D-Day stories to mark the occasion.
In the afternoon, tributes will move to the beaches of Normandy, where hundreds of allied defence personnel will parachute into a historic D-Day drop zone to commemorate the airborne invasion of 80 years ago.
The Royal British Legion will host a commemoration service at Bayeux War Cemetery before the Ministry of Defence and the Commonwealth War Graves Commission host a joint UK-France thanksgiving service at Bayeux Cathedral.
Tonight, as the sun sets, the Commonwealth War Graves Commission's Bayeux War Cemetery will light up in honour of those who fought on the beaches. The light show will be streamed live on the BBC so all can witness the occasion.
Akshata Murty, wife of Prime Minister Rishi Sunak, next to Sir Keir Starmer and Johnny Mercer at today's D-Day commemorations
Sir Keir Starmer poses for a photo at the D-Day commemoration in Portsmouth today
David Cameron sips from a can behind Sir Keir as the nation prepared to commemorate the D-Day landings
Johnny Mercer - the minister for Veterans' Affairs and a local MP - chats with Mr Cameron in Portsmouth
A veteran looks on as people gather to attend today's national commemorative event
A Normandy veteran watches on as he prepares for the commemoration to begin
Members of the military on stage during the UK's national commemorative event
On Thursday - the 80th anniversary of D-Day - commemorations will begin in Normandy at 7.25am, the same time the beach invasion began in 1944.
A military piper will land on the beaches of Arromanches-les-Bains in a Royal Marines landing craft and begin playing a lament in tribute to those who led the beach landings.
The official British commemoration for the 80th anniversary of D-Day will take place at the British Normandy Memorial at Ver-sur-Mer, where the King will join French President Emmanuel Macron and Mr Sunak.
Charles and Camilla hosted four D-Day veterans at Buckingham Palace yesterday, hearing moving personal stories and seeing their poignant keepsakes.
Liberal Democrat leader Ed Davey chats to a fellow attendee at today's event in Portsmouth
Defence Secretary Grant Shapps speaks with service personnel at the event this morning
Lone bagpiper Richard Cowie, from the Cumbria Freemasons, warms up his pipes ahead of playing at a D-Day commemoration event on Windermere, Lake District
Mr Cowie will be playing on board one of Windermere Lake Cruises traditional steamers at Bowness-on-Windermere
Planes fly overhead the Standing with Giants installation at the Normandy British Memorial, in Ver-Ser-Mer, France
The memorial features 1,475 silhouettes across the wild meadow fields of the British Normandy Memorial
Football boots carried on the straps of a military backpack, dog tags still bearing blood, and photos of a much cherished wife were among the mementoes shared with Charles and Camilla.
Charles, in turn, read aloud from his grandfather's handwritten diary, recounting George VI's D-Day entry about the breaking news of the 'successful landings' in June 1944.
The special meeting was filmed and will be broadcast as part of BBC One's D-Day 80: Tribute to The Fallen on June 5.
John Dennett, 99, from Liverpool, gets emotional during a Spirit of Normandy Trust wreath-laying service just off the French coastline
Veteran Donald Jones reacts as he returns to Sword Beach in Normandy
D-Day veteran Alec Penstone, 98, is pushed past crowds at the Spirit of Normandy Trust service
Mr Penstone is joined by his comrades Stan Ford, 98 and John Dennett, 99, today in France
Ken Hay, 98, (far left) and Alec Penstone, 98 (second left) at the statue of Field Marshal Montgomery during the Spirit of Normandy Trust service
Mr Penstone takes his place alongside his comrades at the ceremony in Normandy
Parachutists wearing replica WWII-era paratrooper attire check their equipment prior to their jump from a Lockheed C-130 Hercules aircraft in Normandy today