The fight happened last Friday in Justin Herman Plaza. The video was shot before police arrived by a woman who sells t-shirts in the area.
People were using hammers, skateboards, and easels as weapons.
KTVU spoke with one witness who said it looked like a "mass riot."
At least four people were hurt by the time it ended and police were on the scene. Two people were taken away by ambulance.
It's not clear how the brawl started, but there are reports that two groups of skateboarders had started fighting and the situation erupted when an artist tried to intervene.