San Francisco tells flash mobs to mind their manners.
The city would also like notice before the next mass pillow fight, cream pie battle, or other messy group antics coordinated by nameless organizers using the Internet or mobile telephone text messages to spread plans.
Officials in a city known for letting almost anything go say they are fed up with group participation shenanigans seen by some as Internet-Age performance art and by others as silliness that leaves streets trashed.
The breaking point came in February, when from 1,500 to 3,000 people showed up for a rainy Valentine's Day pillow fight that left a downtown plaza and surrounding streets caked with feathers.
It was the fourth year of the event, which started out as a flash mob stunt but turned into an annual ritual orchestrated mysteriously using the Internet and mobile phone messages.
The flash mob phenomena dates back to 2003 and essentially consists of large groups of people assembled on short notice to briefly join together in doing odd things and then dispersing as though nothing unusual happened.
Mob pillow fights have taken place in numerous cities, with flash mobs around the globe taking part in a World Pillow Fight Day in 2008.
Last week a flash mob fought playfully with shaving cream pies at the spot downtown where the famed cable cars turned around and tourists queue to board.
Online notices for the San Francisco pillow fight told people to spread the word to everyone they know, bring a pillow hidden in a bag, and "practice responsible fun and help clean up."
Feathers are blamed for clogging drains at nearby restaurant, office building, and public fountain. A city clean-up crew reportedly carted away three truckloads of feathers while cleaning the fountain.
Cleaning up after the pillow fight, which went on for about six hours, was said to total more than 30,000 dollars.
San Francisco officials are threatening to ban such events if organizers don't follow the rules when it comes to getting permits and tending to security and clean-up.