A naked man who mounted a city billboard armed with a small pistol has been arrested by heavily armed police yesterday.

Three officers from the Police Tactical Response Group used a "cherrypicker'' crane and finally managed to bring the man down to the ground, where he was taken to a waiting ambulance.

The drama ended just on 6pm, after nearly four hours.

The man is in police custody and has been taken to Royal Perth Hospital.

Police were unaware if the man was brandishing a genuine pistol or a replica, but later confirmed it was a genuine weapon.

The bizarre incident brought central Perth to standstill when the man climbed on the advertising billboard on the corner of Wellington and Barrack Streets, about 2pm.

For nearly four hours the naked man had been at the centre of a major shutdown of streets in the CBD, which is also caused disruption to city train services.

Earlier, Central City Medical Centre staff were eye level with the gunman when they were trapped in their office less than 100m from man.

About 5.50pm, receptionist Jamie Chan, 24, of East Perth, heard the police call to the man: "'We're here to help you, please come down here. We can get you suitable medical attention."

The man, who appeared to be in his 30s or 40s, had been yelling at onlookers and was pacing up and down the billboard, approximately 10 metres in the air.

No shots have been fired.

Heavily armed Police and Tactical Response Group officers were called and police snipers could be seen taking position in a nearby carpark.

Parts of Wellington, Barrack and Stirling streets were closed to traffic and pedestrians, with detours in place.

A Public Transport Authority spokesman said the incident had caused “major disruptions” to public transport services in the area.