PHILADELPHIA — 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 here 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.

On Wednesday, the police here said that they had had enough. They announced plans to step up enforcement of a curfew already on the books, and to tighten it if there is another incident.

They added that they planned to hold parents legally responsible for their children’s actions. They are also considering making free transit passes for students invalid after 4 p.m., instead of 7 p.m., to limit teenagers’ ability to ride downtown.

“This is bad decision making by a small group of young people who are doing silly but dangerous stuff,” Mayor Michael A. Nutter said in an interview Wednesday. “We intend to do something about it immediately.”

Flash mobs are not unique to Philadelphia, but they have been more frequent here than elsewhere. Others that resulted in arrests and injuries have been reported over the past year in Boston, South Orange, N.J., and Brooklyn.

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