LINDA WERTHEIMER, host:
They're known flash mobs - gatherings of young people who use social media and texting to publicize a spontaneous group dancing at a train station or get a big crowd together to imitate statues in a park.
But in Philadelphia, flash mobs have turned violent, and Susan Phillips of member station WHYY reports that police are cracking down.
SUSAN PHILLIPS: South Street is a popular hangout for kids and tourists. Two weeks ago, pizza shop owner Joey Rocco was finally getting some decent business after a long snow-packed winter.
Mr. JOEY ROCCO (Owner, Joey's Fire Stone Pizza): Saturday night we had the windows open; it was a beautiful night. People were setting in the windows areas. I just happened to look out and I said, wow, the street is really crowded.
PHILLIPS: Some say the crowd was in the hundreds; others say thousands. Rocco says the kids began to jump up and down, and then utter chaos broke out. He says some of the teens started beating each other up, while others began banging on the windows of his shop.
Mr. ROCCO: They were trying to climb in through the windows on top of the people who were dining in the area. So, we pushed them out and closed the doors and then I locked the front doors. So, whatever they had in mind, to me, was like a home invasion.
PHILLIPS: And it wasn't the first violent flash mob for Philadelphia this year. Two other incidents had kids pushing down pedestrians outside city hall and pouring into the Macy's department store, where they tore merchandise off the shelves.
Of the 30 kids picked up and charged in those earlier incidents, 29 got hit with felony convictions. After the third incident, Mayor Nutter stood with a phalanx of police near South Street and warned parents to get better control of their kids or face their own charges.
Mayor MICHAEL NUTTER (Democrat, Philadelphia, Pennsylvania): Parents have a responsibility here, so we will do all that we can. But I ran for mayor, I didn't run for mother. So, you know, I can't take care of everybody's child.
PHILLIPS: Nutter says he will extend the curfew if the flash mobs continue. One state lawmaker wants to bring back the mounted patrols. And police have created a special rapid response force.
Acting on tips, officers converged on several areas after the last incident, but found no flash mobsters. Police were out in force on South Street last weekend, which ended up being a lot colder than the week before.
Unidentified Man: How are you doing, sir? (Unintelligible).
PHILLIPS: The city's new district attorney, Seth Williams, took a walk down the popular shopping street with neighborhood leaders. Merchants, parents and police are scratching their heads trying to explain the explosion of violent flash mobs.
So how does it all happen? Williams says it comes down to the proliferation of computers and cell phones.
Mr. SETH WILLIAMS (District Attorney, Philadelphia): And the messages get spread geometrically, you know, exponentially. Where before it was just, you know, you tell someone to meet you somewhere or if there was a fight, meet them at the schoolyard. Well now, instead of just 10 or 20 kids maybe at most knowing about something, you've got a thousand kids showing up at an intersection and that's a problem that we, as law enforcement, have to try to deal with.
PHILLIPS: Fifteen-year-old Benjamin Hamilton(ph) says he's never participated in a flash mob, but he knows kids who have.
Mr. BENJAMIN HAMILTON: They talk about it like it's a game, or like it's, like, just a night of fun. And really, it's not, because, like, it's really affecting people in ways that they can't imagine. They might be affecting somebody who might get injured or might get hurt or might get emotionally hurt.
PHILLIPS: Hamilton says kids he knows dismiss the crackdown and the increased curfew.
Some teens spoke out on a local radio show about the incidents. But after one was targeted as a snitch, he was too fearful to speak. The kids said the flash mobs were organized by party groups and some speculate that gang members took advantage of the chaos.
It's been more than a week since the most recent violent flash mob sprung up, but police are concerned about this weekend, where temperatures are expected to be in the 70s.
For NPR News, I'm Susan Phillips in Philadelphia.
NPR transcripts are created on a rush deadline by a contractor for NPR, and accuracy and availability may vary. This text may not be in its final form and may be updated or revised in the future. Please be aware that the authoritative record of NPR's programming is the audio.