City Beach attack: Perth man Trae Black has his leg severed after allegedly being run down by a car and attacked with a hammer in horror assault

A young man has lost a leg and is now fighting for life after a colleague allegedly chased him down in a car, ran him off the road and attacked him.

Trae Black, 29, was allegedly attacked on the West Coast Highway, in Perth's City Beach, on Monday morning.

It is understood the pair were involved in a heated altercation at a construction site several hundred metres away in the beachside suburb before a pursuit began, ending with Mr Black's leg being severed below the knee. 

Emergency crews and bystanders attempted to help Mr Black, who was allegedly run down by the car before being struck in the face with a hammer.

A retired army veteran was one of the first bystanders to help Mr Black, who he said 'wasn't doing too well' after the incident. 

'He was asking for water and was quite alarmed,' the Good Samaritan told 9News. 

'I later learned that there was a male who (allegedly) had hit him in the face with a hammer and was also on the scene,' 

'Later on I found [the victim's] amputated leg up on the bank and put that in a bag and provided that to paramedics.'

WA Police arrested a man, 25, nearby a short time later.

He is assisting police with inquiries. No charges have been laid.

It's understood the men were known to each other.

Mr Black was rushed to Royal Perth Hospital in a critical condition, where he underwent emergency surgery for his injuries. 

Superintendent Manus Walsh told the reporters that the alleged incident was sparked by a 'verbal altercation' at a nearby construction site.

'The victim ran from the area and was (allegedly) chased by another man in silver Mazda utility,' Supt Walsh told reporters.

'It is possible that the victim was struck by the vehicle and a secondary assault has then occurred by the offender with the use of a hammer.'

Supt Walsh thanked bystanders who rushed to Mr Black's aid, including an off-duty orthopaedic doctor who used her handbag straps as a makeshift tourniquet for him. 

'We are very, very thankful for the off-duty doctor,' he said.

'This was a harrowing incident for anyone to observe, whether they be trained police officers, where they be members of the public.'

Supt Walsh added it was likely Mr Black would have died without the help of bystanders.

The West Coast Highway was closed in both directions and traffic was diverted while detectives and forensic officers scoured the scene.

Investigators also cordoned off a large blood stain on the road. Torn clothing was also strewn across the highway

Police divers were seen scouring the shoreline of a nearby beach in search of a weapon which was allegedly disposed of near the scene. 

Witnesses described the weapon as looking like a demolition hammer.

One witness who overheard the original scuffle told the publication that he overheard someone threatening someone else. 

'I thought I heard him say I'll catch you, or I'll get you, or something like that,' he said.

Police are appealing on anyone with information, CCTV or dashcam footage to contact Crime Stoppers.