A swarm of bees has descended on one of Perth's main city streets, with a number of shops closed as beekeepers try to manage the situation.

The bee swarm has been in the pedestrian mall since about lunchtime.

A beekeeper called in by the City of Perth estimated there were more than 20,000 bees in the swarm.

"It was the largest colony I've seen this season," Mike Conway from Bivouac Beekeeping said.

"It was about 2 metres up the wall by a metre wide."

An area of Murray Street was closed to foot traffic because of the swarm. ( Twitter: Dilettantiquity )

Mr Conway said it was hard to say why the swarm chose that location.

"You would have to talk to the bees about that one," he said.

"Looking for a home in the CBD is not too easy for them.

"I wouldn't say it's common but it does happen in the City of Perth.

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"There is so many people keeping bees in very unusual places including the CBD that there is bees everywhere now."

City retail worker Liria Siragusa said things became "hectic" as people failed to notice the swarm until they were in it.

"It looked like a big cloud of black, like the rapture," she said.

"It was great to see everyone's reactions, everyone was having a good time, ducking through it [and] having a laugh."

The City of Perth advised in a statement that anyone with bee allergies should avoid the area while the last of the swarm is moved on.

It was reported to the city about 1:40pm.