SFPD Police cracking-down and arresting many peaceful revelers during the Bay to Breakers footrace festival. It did not appear that the people they arrested were drunk or causing any trouble. They got arrested just because they were partying in the street. Police apparently had orders to crack-down on all festivities on city streets at the 2011 Bay to Breakers race.
SFPD Police did not arrest people that were partying on Park land (Pan handle and Golden Gate Park). I suppose that the rules for arresting people on Park property are a bit different.
Photo taken at the 2011 Bay to Breakers footrace and street festival (San Francisco).