SAN FRANCISCO (CBS / AP) — A throng of cyclists hit the streets of San Francisco Friday night, as thousands of people commemorated the 20th anniversary of the raucous, leaderless ride and street party known as “Critical Mass.”

San Francisco police monitored the ride and recommended that people avoid driving in downtown San Francisco Friday before the riders left Justin Herman Plaza at 6 p.m. to take over city streets.

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The monthly event has inspired similar rides in other cities across the globe, and participants in Friday’s anniversary ride came from as far away as Italy and Mexico. Critical Mass also helped spur a suite of city policies that encourage bicycle commuting, including more bike lanes and bicycle parking spots.

The San Francisco Municipal Transportation Agency said ridership grew 71 percent from 2006 to 2011 alone.

Organizers say Critical Mass is not only a time to enjoy urban spaces via bicycle, but also to “challenge the use of city streets and the domination of cars and oil in our transportation system.”

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Several anniversary events, including a film screening and a book release party, were held around the city this week leading up Friday night’s ride, which started at Justin Herman Plaza at the foot of Market Street.

A special “Kidical Mass” contingent was set to take part in the ride, as well as a women’s contingent organized by a female cycling group, “A Girl and Her Bike.”

The anniversary celebrations will continue throughout the weekend including a Sunday 1 p.m. “Farewell Bike Ride” leaving from 518 Valencia St. to head to a party at Ocean Beach.

For more information about Critical Mass and its anniversary events visit

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