After finding 10 teenagers guilty of felony rioting and conspiracy in Family Court yesterday, Judge Kevin Dougherty said he was determined to get to the bottom of the "flash mob" phenomenon.

"I'm not playing," he warned the juveniles. "The days of civil unrest are over. Philadelphia must heal."

Dougherty asked each of the teens why they had gone to Center City on Feb. 16, when a huge gathering of students turned into an ugly melee. The first several said they had gone downtown to go shopping or meet girlfriends.

"You all want to play me and say you went down there shopping," Dougherty said. "I'm not believing you."

In one exchange with a 15-year-old Simon Gratz High School student, Dougherty told the boy he would get a year in the juvenile system for every lie. By the time the boy was taken away in handcuffs, he had received three years.

In the end, several said they learned that students were going to gather that afternoon at the Gallery at Market East - and that some groups were going to fight - through the social media sites My Space and Facebook.

Despite getting the information he wanted, Dougherty seemed determined to send a message about the flash mobs, which first sprang up in May.

The most recent incident happened on Saturday night, when hundreds of teens stormed down South Street, forcing businesses to close. A huge police contingent had to disperse the crowd.

"The lesson these kids are going to learn today is . . . the days of being slapped on the wrist are over," Dougherty said.

In the initial incidents in May and December, those arrested faced mostly misdemeanor charges. The teens and young adults arrested on Feb. 16 and after another bout of Center City troublemaking on March 3 were all charged with felonies.

The juveniles arrested in the March 3 incident are scheduled to appear before Dougherty today.

Yesterday, 15 juveniles were scheduled to appear before the judge. Four agreed to plead, but their hearings were delayed. Of the 11 who remained, Dougherty found 10 guilty.

Just one was allowed to leave the courtroom on home detention. Dougherty delayed his decision on three others who showed some promise - one had signed up for the Pennsylvania National Guard before his arrest - but he ordered them into custody while he determined their ultimate fates.

The rest faced punishments ranging from 30 days in boot camp to committment to a state juvenile facility.

When citizens are afraid to go downtown "because Philadelphia children are terrorizing them, I will make a decision to remove you from civilized society," Dougherty said.

Because those involved are juveniles, The Inquirer is withholding their names.

One 15-year-old Gratz student took the blame for sparking the large gathering at the Gallery. He said he is involved in a dance group called Team Nike and that mall security guards let the teens dance there.

On Feb. 16 he said he put a "friend blaster" message on MySpace that the dance group was headed to the Gallery to make a video.

"Anybody who thinks this is a flash mob, it's my fault," the teen said. "This time, it was too many people . . . They ain't flash mobs. They're dance groups."

Another 15-year-old Gratz student also said that informal, neighborhood dance groups are at the heart of the gatherings. He said the groups throw parties and sometimes identify themselves with homemade shirts and hoodies.

But the student also said he expected trouble that afternoon.

"I went down there mainly because I heard a lot of people talking about a fight," he said. "I went down there to see what happens."

A gathering estimated to be as large as 150 teens were expelled from the Gallery. Some teens then rampaged through the nearby Macy's, fighting and breaking items.

Another large group convened at 15th and John F. Kennedy Boulevard at rush hour. One teen was arrested for throwing an "iceball" into the street.

That crowd also filled the concourse at Suburban Station, where several were arrested.

Officer Louis Pagan, who made three arrests that day, said the crowd was so overwhelming that police broadcast the highest level of "officer assist."

"That assist usually goes out when there's an officer down," he said.