Roughly 1,000 people drawn by internet postings and word-of-mouth converged near San Francisco's Ferry Building on Tuesday night for a half-hour pillow fight.

The underground event erupted at 6 p.m. in the center of Justin Herman Plaza with a mass rush of shrieking, laughing combatants - many of whom arrived with pillows concealed in shopping bags, backpacks and the like.

Within minutes, pillows were arcing, feathers were flying, and by the time the Ferry Building's clock tower clanged the half-hour, the plaza and hundreds of people were covered in white down that gave the scene a wintry lustre.

"I haven't giggled so hard for a really long time,'' said San Francisco resident Amy Davis, 35, an office manager for a construction company that manufactures stone facades for buildings.

Davis - who said she has been unlucky in love and was grateful for an antidote to Valentine's Day -- lasted for most of the battle, but pulled out toward the end when she had her fill of breathing feathers.

Like many others, Davis learned of the pillow fight from a friend who directed her to a web site - in her case it was Wikipedia - that gave details about a planned flash mob pillow fight on Valentine's Day in San Francisco.

The rules for the affair were simple: tell everyone about the upcoming pillow fight, show up with a concealed pillow, and don't hit people without pillows.

Flash mobs rely largely on the internet, spreading word of events via email, chat rooms, and text messaging.

It was San Francisco's first "organized'' public pillow fight, but such events have occurred in other parts of the country and the world.

"It's just a meme. A meme is when a thought goes out and becomes part of consciousness,'' said Amacker Bullwinkle, a Palo Alto artist with purple streaks in her hair who claimed affiliation with a mysterious group called the Pillow Fight Club.

Bullwinkle came with a camera and because of a knee injury mostly stayed on the edge with about a third of the crowd who watched as hundreds of others beat the stuffing out of each other.

"No injuries, no cops and lots of smiles,'' Bullwinkle said after it was all over.

Bullwinkle did not know who organized the pillow fight, but she said many in attendance were veterans of Burning Man, the annual art-party extravaganza in Nevada's Black Rock Desert.

But not Blitz Ono, a resident of Tokyo who has been an undergraduate at UC Berkeley for the last two years. Ono, 22, heard about the event from a friend and came, pillow in hand, not knowing what to expect.

"I was surprised. I didn't think people would get that serious into it,'' said an elated Ono, who was swinging the whole 30 minutes.

"I had so much fun. It was great. It was amazing,'' he said.

Edwin Coreas, 22, a San Francisco cable television installer, was equally pleased, and glad that he found another occasion to wear the Halloween costume he bought last year that turned him into a human kissing booth.

"I'm really surprised that there are this many people,'' he said, staring in amazement at the furiously swinging mob as he clutched a couch pillow in his left hand.

"This is pretty great.'