After online rumors stoked fears yesterday of yet another potential flash mob - this time at 40th and Market Streets - police told businesses there to close, parked cruisers in the middle of street, and stationed officers at each corner.
No large, destructive group of teenagers materialized. Still, the mobilization showed the city's heightened sensitivity to the phenomenon of flash mobs, which have struck Center City and South Street four times since December, fueling worries that the gatherings are harming businesses and the city's image.
Hours earlier, Mayor Nutter, flanked by about 40 uniformed officers and joined by the police commissioner and district attorney, had held a news conference in Headhouse Square to announce, again, that the city would not tolerate misbehavior.
Nutter punctuated his dual messages - to reassure the public and to castigate parents for their lack of supervision - with exasperation when asked what preventive measures the city could take and what he thought was motivating the teens.
"I ran for mayor. I didn't run for mother," Nutter said. Later, he added, "I don't know what causes someone to act like a jackass."
The news conference came after two days of Family Court hearings into flash mobs in February and March. Those hearings ended Tuesday with 28 juveniles convicted of felony rioting.
Half the students were taken into custody, and all received a verbal scalding from Kevin Dougherty, the court's administrative judge.
"The days of being slapped on the wrist are over," he declared Monday.
The sizes of the seemingly spontaneous gatherings have varied, with estimates well into the hundreds. The teens have fought among themselves, assaulted pedestrians, and smashed up the downtown Macy's store. One gathering in February was so out of control that police called for an "assist" that normally is broadcast when an officer is shot.
The latest gathering happened Saturday, when hundreds of teens stormed South Street, forcing businesses to shutter.
Nutter and Police Commissioner Charles H. Ramsey said yesterday that police would assign more officers to trouble spots.
Nutter also said he was prepared to sign an executive order making the city's curfew for minors earlier, though he would not say what the new time would be. He noted, however, that three of the flash mobs occurred shortly after school, well before the current weeknight 10:30 curfew.
Nutter and Ramsey encouraged parents to monitor their children's online activity and text messages. Parents tipped police to Saturday's South Street incident and yesterday's potential gathering at 40th and Market Streets.
"It's not the government's responsibility to raise your child. It's your responsibility," Ramsey said. "When we get involved as police, it's too late for the tears."
Mike Shafi, manager of the Crown Fried Chicken at 40th and Market, said teens regularly packed his store after school. Several businesses pulled down their gates yesterday afternoon, but Shafi's remained open.
"The police told me to be careful," he said, "but I don't know. I've never had problems."
The crowd that showed up for the supposed gathering consisted mainly of news reporters. Sgt. Charlie Marsden, who works out of the University City substation, was assigned to one corner.
"We had a group of girls show up," he said. "They wanted to watch what happened. Thought it was going to be a big event."
Marsden shooed them away.
Melanie Kennedy, a student-support staff member at nearby Paul Robeson High School, said talk of the flash mob had circulated through the school yesterday.
"We told the kids to go home after school, but they hear that and they just want to run right to the trouble," she said. "To them, it's some fun, just something to do. They figure the police can't arrest all of them."
Word of the gatherings has spread through text messaging and social-media sites such as Facebook, but the online organization could be nothing more than the modern-day equivalent of passing notes in class.
The incidents have happened near the Gallery at Market East, the CVS at 15th and Chestnut Streets, and along South Street - places that have attracted large crowds of teenagers for generations.
The latest South Street incident occurred during the first warm weekend night of the year, when large crowds of teenagers and young adults could be expected there.
Dougherty questioned most of the juveniles about the role the Internet played in organizing the flash mobs. Several claimed to have been in Center City for other, legitimate reasons, but some admitted to learning of the gatherings through social media or texts.
Two said they expected trouble downtown, including one who said he had gone to the Gallery specifically to rumble with rival students from Bartram High School.
Two teens - The Inquirer is withholding their names because of their ages - also said informal, neighborhood dance groups with a presence on MySpace had been at the heart of the gatherings.
One dance group identified in court, Team Nike, posted a video on YouTube this week showing about a dozen teens on South Street dressed in what appeared to be homemade Team Nike shirts. The teens dance and shout slogans, but are peaceable throughout.
At one point, the message "We got tha whole South Street following us Team Nike" flashes on the screen. The team members are, in fact, shown at the head of a large crowd.
Nutter and others have described the flash mobs as populated largely by good kids simply looking to hang out. A small percentage of "knuckleheads" bent on violence have been responsible for the trouble, the mayor said.
Whether the flash mobs are intended to cause mayhem or result from too many unsupervised teens in one place also isn't clear.
Ramsey said he didn't think the phenomenon was exclusive to Philadelphia, but he couldn't provide examples from other cities.
He said the extra deployments to deal with the flash mobs were a drain on department resources that could be used on something other than "chasing . . . youngsters."
Ramsey also urged parents to get control of their children, and asked anyone with information on possible flash mobs to call 215-686-TIPS (215-686-8477).
"We are not baby-sitters," he said. "That's just not what we get paid to do."
Contact staff writer Troy Graham at 215-854-2730 or email@example.com.