Posted by 
A man is in a critical condition and undergoing surgery after one of his legs was severed in an alleged attack by a work colleague.
The remains of the leg were found by a good Samaritan on the side of the road in coastal Perth.
Traffic in City Beach was brought to a standstill as the victim fled on foot from a nearby construction site.
Trae Black, 29, was allegedly hunted by a work colleague in a Mazda ute and hit.
His right leg was severed below the knee.
The West Coast Highway was covered in blood after the alleged attack.
Nick Black was among those who stopped to help about 11.20am.
"He was asking for water and was quite alarmed," he said.
An orthopaedic doctor used her handbag strap as a makeshift tourniquet – and later found the remains of his leg.
"Later on I found his amputated leg up on the bank, put that in a bag and provided that to paramedics," she said.
Police say the passersby who helped likely saved his life.
The highway was closed in both directions as dozens of police investigated.
It all started several hundred metres away at a construction site on Catesby Street in City Beach
It's understood Black fled on foot when a workmate from another site came looking for him.
Police said the two men, who knew each other, had been involved in an argument.
The other man allegedly also took to the windscreen of a car parked out the front with what witnesses have described as a demolition hammer.
That tool is missing and police divers searching the ocean for it.
"It is believed the victim was struck by the vehicle before being assaulted with a hammer," WA Police said in a statement.
A 25-year-old man was arrested near the scene less than 15 minutes after police were called.
Investigators are now calling anyone who may have dash cam vision of what unfolded to come forward.
Watch every moment, every medal of the Olympic Games Paris 2024 live and free on Channel 9 and 9Now. Plus, every event ad-free and in 4K on Stan Sport.
