A man was injured after being dragged 20m by a car in North Perth at the weekend.
The 27-year-old victim and a 24-year-old female friend were walking two dogs at a reserve on the corner of Namur and Fitzgerald streets at 10.30pm on Sunday.
"The woman parked her vehicle nearby using the headlights to see in the darkness," police said today.
A man got into the driver's seat and started to drive away as the dogs also got in the car.
"The woman tried to stop the offender from driving away by reaching inside the car to disable the ignition," police said.
"The offender then got out of the vehicle and threatened the pair, claiming he had a knife."
The attacker punched the male victim in the face several times before returning to the car.
"Both victims were further assaulted by the offender as they eventually managed to rescue the dogs from the car," police said.
"The male victim was also dragged by the moving car for approximately 20m, suffering a broken ankle.
"The offender then drove away at speed in the woman's BMW Coupe, which was since recovered."
Western Suburbs detectives have called for the public's help to find the offender, who is described as aged between 25 and 28 years.
He has a dark complexion and blue eyes and is about 180cm tall with a medium/athletic build.
He was wearing a black hooded jumper with white writing on the front, and dark-coloured loose-fitting pants.
Anyone with information about the offender is asked to call Crime Stoppers on 1800 333 000.