The party began as soon as Kevin Durant, Stephen Curry and the Warriors beat the Cleveland Cavaliers 129-120 at Oracle Arena to clinch their second championship in three years Monday night.
In San Francisco, people hung out of cars waving blue and gold flags.
Across the bay, rowdy crowds gathered in Oakland intersections, including one were where fans cheering the championship made a circle to watch several men breakdance. Some climbed street lights, some sprayed champagne and some threw bottles leaving broken glass in streets. Others were setting off fireworks.
Spontaneous celebrations were breaking out in streets around the city.
There were no immediate reports of any violence or arrests, but skirmish lines of Oakland police were trying to corral the partiers and keep them from spreading too far.
The Warriors won the title in 2015 before the Cavaliers made their historic comeback last year. Then it was Golden State’s turn again, taking the title in five games.