A man is in custody in Perth after what police have described as a firearm incident.

Western Australian Police say the man was detained in a vehicle on the Graham Farmer Freeway on Monday.

The incident prompted police cordons in the area around Lord Street, Wittenoom Street, Brook Street and the East Parade area, causing traffic delays.

Motorists have been advised to avoid the area but police say there is no longer any threat to the public.

-AAP