A SIEGE is under way at the building housing the Perth stock exchange and commonwealth parliamentary offices, where a man is making threats from the building's roof, police say.
Police believe the man may be armed.
Tactical response police have cordoned off part of the CBD.
Sergeant Graham Clifford said officers secured the area and began searching for the man after reports of an offender with a gun at 12.20pm.
A witness said police were everywhere.
"There are heaps of cops, unmarked cars and plain clothed cops - stacks of them,'' the woman told PerthNow.
"They wouldn't tell us anything, so I'm just driving around and around. I can't even get into work.'' - With AAP
