WA police have closed off the entry to Perth's main train station and surrounding city blocks while they negotiate with a naked man who's armed with a gun.

Police say the man is acting irrationally while standing on top of a billboard at the corner of Wellington and Barrack Streets in the centre of Perth.

The tactical response group and police negotiators are speaking to the man. Negotiators are using a cherry picker to reach the man.

All train lines are experiencing delays and southbound commuters on the Midland and Armadale lines are being diverted to McIver train station out of the city.

Duty Police Commander Inspector Neil Blair says various parts of the city will be closed off for sometime.

"We've cordoned off between one and two city blocks to all vehicle and pedestrian traffic," he said.

"We believe we're just outside of firing range now and we've had to isolate carparks, hotels, as well as the railway line.

"There's no trains running south of the Perth's city station and that'll be indefinite at this stage."