Philadelphia implements curfew to curb violence

>>> thousands of extra police officers are flooding the streets of london tonight in the first weekend since violent riots broke out across the city. police say they've made more than 2,000 arrests charging more than 700 people after a week of the worst violence there in decades. five people were killed as rioters looted hundreds of stores and set buildings and cars on fire. meantime back in this country, the city of brotherly love is dealing with a rash of violent assaults on its streets. philadelphia police say they've arrested dozens of teams. last night, the first night of a mandatory weekend curfew for minors. the curfew is designed to prevent flash mobs of young people responsible for a string of random attacks. we get our report tonight from nbc's ron allen .

>> reporter: the attacks seemed random, a roving gang in philadelphia beats up a man just walking a downtown street.

>> hi, my mom just got like attacked by a flash mob . regi

>> reporter: anguished calls for help at a state fair as marauding teen leave smashed cars and stunned victims in their wake. the swarms of mostly young people organize through social media , texting, tweeting, using internet sites . and increasingly turning violent in cities across the country.

>> reporter: flash mobs were once known as gatherings of performance artists for fun, games. now experts say flash mobs are emboldened by the anonymity of the internet and a mob mentality .

>> a lot of young people who feel that they're very disconnected from the society. but the one thing i do have is a cell phone.

>> this nonsense must stop.

>> reporter: philadelphia's mayor delivered that warning after two flash mob attacks in two weeks. in the latest incident, as many as 30 young people beat and robbed bystanders in center city . four people were arrested, including an 11-year-old.

>> parents, get your act together. get it together. [ applause ]

>> reporter: emily gindlesburger spent ten days hospitalized after an attack. with her leg broken in two places.

>> we had like a lot of really angry kids who don't really, you know, have any respect for other people.

>> reporter: extra police now patrol downtown, and enforce a strict curfew keeping teens out of certain areas after 9:00 p.m . other cities like chicago also are trying to stop the violence using teen curfews. here in new york, police have a brand-new unit that patrols on line, scouring social media looking for trouble. fighting crime that's sudden, often brutal, and very much a sign of our digital times. ron allen , nbc