At least two people were killed and up to 100 people injured when a passenger train carrying 800 people collided with a truck, derailing eight carriages, Russian media outlets reported.
Dramatic pictures show how carriages lay twisted and battered beside the railway track after the collision in the southern Volgograd region near the Kotelnikovo station about 1,200 km (750 miles) south of Moscow.
The train had been travelling from Kazan in Tatarstan to the holiday resort of Adler on the Black Sea when it derailed, the emergencies ministry said.
'Paramedics are on the scene,' the ministry said, adding that 324 emergency workers were involved in the operation. Emergency helicopters have also been scrambled, it said.
Passengers were trapped inside the carriages while those who had been able to escape tried to free them before emergency services arrived at the scene. Some were wedged under the train.
Passengers were trapped inside the carriages while those who had been able to escape tried to free them
Dramatic pictures show how carriages lay twisted and battered beside the railway track after the collision
Shocked onlookers gather around the train as people rushed to help those trapped inside
The mangled train carriage is seen after the collision
The Mash Telegram channel said at least two people were killed but that the death toll could rise. Interfax news agency said that up to 100 people could be injured.
Unverified video on Telegram showed at least four carriages derailed, some twisted, with people climbing out of one that was on its side beside the rails.
Mash published video showing the battered remains of the cabin of a Kamaz truck beside the derailed train.
TASS cited unidentified sources in Russian law enforcement as saying that 20 people had been injured when the train collided with a Kamaz truck.
Other outlets said that initially 37 people, including six children, were injured in the train crash, with four hospitalised.
Distressing footage of carriages lying on their side was posted by passengers from the scene.
The train was en route from Kazan to Adler, part of the Sochi tourism hub on the Black Sea, a 47 hour journey.
The seriously wounded lorry driver Aslambek Usmatov, 43, was rushed to hospital with serious injuries.
Footage showed him wounded and confused after the crash, close to the wreckage of the cabin of his truck which was 50ft from the rest of his vehicle.
Some 324 Emergencies Ministry specialists and 109 pieces of equipment are working at the passenger train derailment site in Kotelnikovo.
Two Mi-8 emergency helicopters were dispatched to the scene.
A criminal probe has been launched.
'All the circumstances of the incident are being established,' said Interior Ministry spokeswoman Irina Volk.
A woman witness said: 'This is a total nightmare.
'A KAMAZ [truck] flew across [the railway line].
'People are trying to climb out, this is a nightmare.'
A second woman said: 'Look, what a nightmare.
'This is our railway crossing in Kotelnikovo.
'Carriages have collapsed, look.
'People are climbing out, what a horror.'
A passenger on the train said: 'A KAMAZ was some 400 metres away as we were approaching the crossing.
'The crew used the emergency brake at a speed of 95kmh [60 mph].
'There was a sharp hit.
'All we could see was a cloud of dust.
'I saw five carriages - one upside down, one on its side, three at 45 degrees.'
He said the train driver and his assistant were alive.'
Last month twom people were killed and 70 injured in a crash involving a tourist train from bleak Arctic mining town Vorkuta to Black Sea resort Novorossiysk.