08/08/2011 04:19 pm ET Updated Oct 08, 2011

Philadelphia Mayor Imposes Curfew To Stop Violent Youth Flash Mobs (VIDEO0

By Dave Warner

PHILADELPHIA (Reuters) - Mayor Michael Nutter announced a youth curfew in Philadelphia on Monday in response to flash mobs that rioted and beat store owners, saying parents would be held as responsible as their out-of-control children.

The crackdown on teens who swarm in response to messages posted on Twitter and MySpace includes a 9 p.m. curfew for anyone under age 18 on Friday and Saturday nights, Nutter told a news conference with Police Commissioner Charles Ramsey.

Parents called to pick up a child who breaks curfew get a warning on the first violation and fines of up to $500 for subsequent violations. Failing to pick up a child could result in a charge of child neglect, authorities said.

"When we get involved, it's too late for the tears. It's too late for 'Oh my baby, Oh my baby'," Ramsey said. "If you love your baby, take care of him now."

Gangs of youths as young as 11 converged on business owners and their patrons in Center City on Saturday night, the most recent in a series of flash mobs in the downtown district.

Among those arrested for beating a man and breaking his teeth was an 11-year-old boy who was "stomping on random people," District Attorney Seth Williams said.

"There are a relatively small number of complete knuckleheads in this city who are causing this kind of disruption, and we will not tolerate it," Nutter said.

In addition to demanding greater parental involvement, the crackdown includes beefed up police patrols and the formation of citizen patrols that can reach police by portable radio in case of emergency.

Nutter said many city recreation centers would also expand their evening hours so youth will have a place to gather for leisure activities.

(Editing by Barbara Goldberg and Cynthia Johnston)