Homicide Squad detectives are investigating the death of a woman whose body was found in a car on the Mitchell Freeway southbound overnight.

The woman’s body was found in a Mitsubishi Lancer being driven on the Cedric Street off-ramp about 12.35am on Saturday, a police spokeswoman said.

A man who was the driver of the vehicle is being questioned and “assisting police with their inquiries”.

In a statement on Saturday afternoon, the police said the woman was believed to be a 35-year-old and a 49-year-old man was helping police.

Detectives were conducting further inquiries at several addresses in Scarborough over the matter.

In video shot last night shortly after the incident, a police woman can be seen standing guard while the man lays on the side of the freeway before being taken away by police.

Police guard a man on the side of the freeway before taking him away to assist with inquiries. Picture: Seven News

That stretch of the Mitchell Freeway was blocked off, becoming a crime scene as forensic officers examine the area at the Cedric Street exit.

The major police investigation is causing traffic chaos on Saturday morning, with diversions in place and motorists urged to take alternative routes.

Mitchell Freeway had been shut altogether southbound between Karrinyup Road in Stirling and Hutton Street in Osborne Park.

A woman's body has been found in a car on the Mitchell Freeway overnight. Picture: The West Australian/Marie Nirme

Just after 8am on Saturday, the freeway section was reopened, but reduced to one lane.

Main Roads warned there was heavy congestion on the road between Erindale Road and Karrinyup Road.

The freeway was later completely reopened.

The Mitchell Freeway southbound between Karrinyup Road and Hutton Street has been closed to traffic.

It’s understood a separate incident occurred in the same area of the Mitchell Freeway around the same time overnight, involving a car crashing into a barrier.

A St John Ambulance spokesman said one person was taken to Royal Perth Hospital under normal road speed following the car versus barrier incident.