Ottawa police are warning of traffic disruptions downtown Wednesday as pro-marijuana demonstrators make their annual spring pilgrimage to Parliament Hill.

Police said they're expecting the demonstration to snarl traffic in the city's downtown between 3 and 5 p.m. on April 20.

Police warned crowds may also cause congestion around Major's Hill Park.

April 20 — or 4/20 as the date is known among marijuana enthusiasts — is the annual celebration of cannabis culture around the world.

The 4/20 moniker dates back to the pot culture of California in the early 1970s, but it became formally attached to April 20 when a group of Vancouver activists held the first day-long rally in that city in 1995.

Rallies have taken place every year since, and have spread across Canada and around the globe.

Two years ago about 2,000 activists gathered — and smoked — on the grounds of Parliament Hill.

That year, good-humoured RCMP officers watched over the scene as clouds of marijuana smoke drifted in the wind.

In 2012, RCMP estimated the crowd gathered on Parliament Hill at nearly 6,000.

Ottawa police said they aren't expecting a larger 4/20 gathering than normal this year, and have no plans to close the roads near Parliament Hill.