WA homicide detectives are investigating the death of a 35-year-old woman after her body was found in a car on Perth's Mitchell Freeway overnight.

Police said the 49-year-old driver of the vehicle was assisting officers with their inquiries.

It is understood the driver of an unrelated car had stopped in the area near the Cedric Street off-ramp after being involved in a crash about 12.30am.

That driver told a news camera operator he was approached by a man who told him he had a body in his car and was being chased by someone.

Inquiries are continuing and the freeway has been opened.

Police said they were conducting further inquiries at several addresses in the northern Perth suburb of Scarborough.