A man has been seriously injured after reportedly being robbed and assaulted by two men in an incident that shut down a Mitchell Freeway exit.

Drivers had to be re-routed at Powis Street in Wembley in the northbound lane of the freeway by traffic incident response and multiple police vehicles.

Police said that the man, aged in his 50s, was approached by two unknown males on Powis Street before being assaulted and having personal property stolen.

St John Ambulance took him from nearby Dodd Street to hospital with severe injuries after the incident about 2.22am.

He was taken to Royal Perth Hospital under lights and sirens, arriving about 3.20am.

He is in a serious but stable condition, a hospital spokesperson confirmed.

Main Roads alerted the closure of the Powis Street off-ramp about 4am. Police were on the scene for several hours.

Detectives were seen working at a scene, with disposable gloves on the ground, at a second location at the Gallup Powis Street carpark.

Two detectives were seen in discussion with a police officer, pointing towards the road.

Police in high-vis trench coats used small torches while walking carefully across the grass next to the off ramp, also combing through the bordering hedges.

Two more police vehicles arrived at the scene shortly before 7am.

The investigation is ongoing and police are asking for anyone with information to contact Crime Stoppers on 1800 333 000 or make a report online here.

Reports can be made anonymously.