Photo by philippe leroyer via Flickr
Flashmobs have been trouble for police in a number of cities for about a year. The New York Times sums them up nicely:
It started innocently enough seven years ago as an act of performance art where people linked through social-networking Web sites and text messaging suddenly gathered on the streets for impromptu pillow fights in New York, group disco routines in London, and even a huge snowball fight in Washington.
But these so-called flash mobs have taken a more aggressive and raucous turn [in Philadelphia] as hundreds of teenagers have been converging downtown for a ritual that is part bullying, part running of the bulls: sprinting down the block, the teenagers sometimes pause to brawl with one another, assault pedestrians or vandalize property.