Around 22 people were arrested at a protest Saturday afternoon at Justin Herman Plaza after projectiles including rocks, paint and flares were thrown at police, according to San Francisco police.
The protest, which began shortly before 3 p.m., started an unpermitted march west on Market Street, according to Officer Gordon Shyy.
Group members, many of them reportedly wearing black clothing and black masks covering their faces, began to block traffic, Shyy said. Some also allegedly vandalized vehicles and businesses as they walked.
When officers arrived on the scene they were struck by projectiles allegedly thrown by group members, Shyy said. One officer received a non-life threatening injury to his head.
Police circled and detained the protestors in the roadway at California and Battery streets, and the protestors allegedly threw flares and bags of paint at officers, some of which contained rocks.
Some protestors also fled to Pine and Sansome streets, and were detained there.
In total, police made 22 arrests, on charges including conspiracy, riot, refusing to obey a lawful order from a peace officer and resisting delaying and obstructing a police officer, and assault and batter on a police officer.
Searches of the protestors' backpacks turned up items including hammers, an ice pick, flares and other weapons and bags of paint containing rocks.
Witnesses are being asked to contact San Francisco police.