A bar in the Perth suburb of Mt Lawley had to be evacuated last night after part of a top floor appeared to collapse.

Patrons in the Flying Scotsman Bar on Beaufort Street say the top floor of the building gave way.

Vincent Mayor John Carey, who was at the bar for the launch of the Beaufort Street Festival, tweeted that no one was hurt.

"In the middle of the launch what sounded like a truck hitting the Defectors Bar saw the Bar shelves smash and the floor appear to drop."