In past years, cyclists with Critical Mass have taken over the Burrard Street Bridge. (CBC)

A planned protest by cyclists Friday afternoon in downtown Vancouver has prompted police to urge motorists to stay away from the city's core.

The Critical Mass bicycle ride is an event held on the last Friday of every month in hundreds of cities across the globe.

In Vancouver, the protest is attended by thousands of cyclists and can cover as many as eight city blocks, taking over city streets and wreaking havoc for vehicle traffic.

The event starts at 6 p.m. at the Vancouver Art Gallery, but the route changes every month.

Insp. Rick McKenna with the Vancouver Police Department said Wednesday motorists should simply stay away.

"We're reminding motorists they should avoid the city core if at all possible. We're encouraging motorists to remain calm and for riders to remain respectful of the rules of the road and the safety laws," he said.

McKenna said the event also has him concerned about trouble.

"Critical Mass was originally designed as a protest over the lack of bike lanes," he said.

"It's now morphed into a major celebration. Unfortunately, it's also morphed into a source of major conflict between riders and the driving public. Last month, we saw an increase in the violence that frequently mars the ride."

McKenna blames both sides for the trouble, but said it could be avoided if the event's organizers would share their plans with police.