City Beach attack: Perth man Matthew Lawson pleads guilty to attempted murder of Trae Black after his leg was severed in a horror attack

A demolition worker has pleaded guilty to attempted murder after severing his colleague's leg when he ran him down with a car before smashing his head with a hammer.

The savage attack at City Beach, in Perth's western suburbs, began when Matthew Lawson, 25, confronted colleague Trae Black, 29 at a Catesby Road worksite on Monday.

Mr Black took off on foot, but Lawson pursued him in a car, chasing him onto the West Coast Highway, where he ran him over and amputated his leg.

Lawson is then said to have jumped out of the vehicle and struck Mr Black in the face with a claw hammer, before fleeing the scene.

Mr Black was rushed to Royal Perth Hospital, where he is currently fighting for life after suffering critical injuries. 

Lawson was arrested near the scene a short time later and faced Perth Magistrates Court on Tuesday charged with one count of attempted murder.

Appearing without a lawyer, Lawson told Magistrate Janelle Scutt 'yep guilty' when asked how he wished to plead, The West Australian reports. 

Magistrate Scutt asked Lawson if he would like to seek legal advice before entering a plea, noting the offence garnered a 'lengthy term of imprisonment', but he declined.

He also turned down the option to have pre-sentencing and psychological reports conducted or the matter adjourned before entering a plea.

He did not make an application for bail and has been remanded in custody. 

The hearing comes as the construction workers' boss, 'Horse' Elliott, revealed new details about the shocking attack.

He said Lawson first turned up at the company's headquarters on Monday morning looking for Mr Black, before seeking him out at the construction site.

'He then came to this (City Beach) worksite looking for Trae ... they’ve had an interaction, Trae’s taken off and Matt’s gotten in the car and chased after him,' Mr Elliott told The West Australian.

'I wasn’t aware of what happened after he left my office until I saw the carnage on West Coast Highway like everyone else.

'But when I came down West Coast Highway...I knew exactly what had happened. He had some fatalistic intentions on his mind.'

Mr Elliott said Mr Black's leg had been severed by the axle of the silver Mazda BT-250 ute being driven by Lawson.

Another employee, who watched the attack unfold, said Lawson - who is just over six feet tall - looked like a 'raging bull'. 

A retired army veteran, who was one of the first responders at the scene, said Mr Black, 'wasn't doing too well'.

'He was asking for water and was quite alarmed,' he told 9News.

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

Mr Black has undergone emergency surgery and remains in a critical but stable condition as of Tuesday. 

Lawson is next due before the Supreme Court on August 21.