NSW man critical after police incident

A man is in a critical condition after he was reportedly pepper-sprayed by police following an altercation in northwest Sydney.

Officers approached the 44-year-old's car on Old Windsor Road at Glenwood about 2pm on Monday after the driver had allegedly smashed into several other vehicles.

The man suffered a heart attack and was taken to Westmead Hospital, ambulance officers say.

An investigation has been launched into what police have declared a critical incident.

"The use of force that was used on the individual is under investigation and we'll be looking at how he was arrested, the manner of his arrest, and that'll all go in the independent report," Assistant Commissioner Michael Corboy told reporters at the scene.

Witnesses are being urged to contact police.