A man is undergoing a medical assessment at Royal Perth Hospital after climbing on to the top of a billboard near Perth's main train station yesterday afternoon, naked and armed with a gun.

Police spent four hours trying to talk the man down.

A cherry picker was used to lift a police negotiator to the man and he was safely brought down shortly after 6:00pm.

All streets around the CBD have been re-opened and train lines into and out of Perth train station are running again.