At the stroke of 6 p.m. several hundred perfect strangers started smacking each other in what has become the annual Valentine's Day pillow fight in Justin Herman Plaza.
"It's like being a child again. You can just hit anyone you want and you get out some aggression," said Ida Hansen, a pillow fighter.
Others engaged in primal scream therapy and there was the thrill of whacking a person, in front of a police officer, without being charged with assault and battery.
"You get a little kick. You can hit someone, but they won't get hurt," said Gro Gausland, a pillow fighter.
It's just pure unadulterated fun. It's just that rush. You can come over here wearing whatever you want and get smashed in the face. Unfortunately, I found that out the hard way.
In the aftermath, there are piles of rain-drenched feathers.
When asked who's going to clean up this whole mess, Hansen said, "I hope it's not me."
San Francisco's Public Works Department says it costs about $17,000 and several days to clean up the mess.
"It's a big inconvenience just because we're going through some very tough economic times right now," said Gloria Chan from the Public Works Department.
But this year, about 30 volunteers were standing by with 200 trash bags. They say they helped clean up a pillow fight in New York's Central Park.
"Last year in New York City we went from having four to six inches of feathers everywhere to having an absolutely clean park," said Jennifer Small, a clean-up volunteer.
Their goal is to maintain these unique uncommercialized social gatherings.
"It was beautiful. It was very therapeutic," said Pascal Jeans-Pierre. When asked what was so therapeutic about it, he said, "Releasing a lot of frustration, anxiety, in a positive way."
The city wants event organizers to apply for a permit and provide cleanup, security and portable toilets. But officials have been unable to track down the organizers.