The 44-year-old man was initially pulled over while driving on Old Windsor Road, Glenwood, at about 2pm on Monday after officers saw significant damage to his car.

It's understood the man was bleeding in his black Jaguar sedan and had failed to stop at an earlier road accident.

When police approached the car, the man got out and started abusing officers.

A critical incident investigation has been launched and officers from Penrith will investigate all the circumstances surrounding the event.

Seven News reported the man was driving erratically and merged from a bus lane, swiping several cars before he was pulled over by officers from the Public Order and Riot Squad.

One witness, Jared Davis, said the man "ran straight at" police once he got out of the car so they tackled him to the ground.

"The guy ran out, and the cops had to do something, so all four of them just jumped on him and got him to the ground but he just kept moving," Mr Davis said.

"As soon as they started doing CPR, it was like 'something isn't right here'. Then he was just still."

Another witness, Jessie Kahale, said the officers "got out with their guns" before they started "laying into" the man.

"They grabbed him and they started laying into him and hitting him, and you know, he was alright but then he became unconscious."

Harley Davis, who was at a service station opposite where the man pulled over, said there was "a bit of a tussle".

"I don't know if it was justified force, but it was just, yeah, it was a bit rough it looked like," he said.

"They had to perform a bit of CPR on him, because he was, he stopped moving for a bit."

The man was taken to Westmead Hospital and remained in a critical condition on Monday afternoon.

Assistant Commissioner Michael Corboy, the North West Metropolitan Region Commander, told media at the scene that the use of force on the man was under investigation.

"We'll be looking at how he was arrested, the manner of his arrest, and that will all go in the independent report," he said.

It is believed one of the riot squad officers who arrested the man was also sent to Westmead Hospital with superficial cuts to his face.

In a statement, police said an "altercation took place" and the man "required treatment by NSW Ambulance Paramedics at the scene."

However, they would not confirm media reports that the man was pepper sprayed, causing him to go into cardiac arrest.

Photos from the scene show the side mirrors hanging off the man's Jaguar, however no other serious damage is visible.

Police are appealing for witnesses to come forward by contacting Crime Stoppers on 1800 333 000.