MOST OF the lemmings who flooded South Street Saturday night - summoned by Twitter - were there to just chill with friends and goof around. We're here, we have no fear, get used to it. Early photos posted on philly.com showed hundreds of kids walking easy, smiling, before the few roaches ruined it.

In other cities, "flash mobs" - the social-media application of punk'd - are large-scale harmless pranks, almost like performance art.

Leave it to Philly thugs to turn them ugly and violent.

Saturday night, in addition to some youths beating each other, a few "mob-sters" assaulted other people enjoying a warm night in Center City.

People assaulted for no reason other than being in the wrong place at the wrong time? And Center City is a "wrong place"?

Fearful business owners shuttered early, restaurants on South Street locked patrons in for their own safety. The rioting embarrassed most Philadelphians, including African-Americans, most especially the mayor, who yesterday promised hard times for out-of-control knuckleheads.

In February there was a rampage along Market East, City Hall and Macy's, resulting in injuries and property damage. (Props to Judge Kevin Dougherty for lowering the boom on those perps.) During a flash mob last May on South Street, motorists were beaten and robbed, a store was looted, passers-by were attacked. A poor guy bicycling home from his night-shift job was critically injured in a vicious attack by eight black males.

Did I say eight black males?

The overwhelming majority of those on South Street Saturday night were black, but also peaceful. Some inexplicable spark ignited the bad characters.

I asked Joel Fein, director of the Philadelphia Collaborative Violence Prevention Center, what might motivate them.

"It's a combination of things," he said. Some act out "because of anger; some may do it to get attention, to look somehow unique among the large crowd."

They also do it because a crowd offers anonymity and protection from prosecution. Some of the violence is "learned behavior," he says.

Fein stressed environmental factors, too, but was loath to blame assaults on racial animus.

Not so the friend of a man assaulted in Broad Street's upscale restaurant Bliss. A teenager, about 16 and wearing a Tiger Woods-style polo shirt, dashed in, punched a man in the head three times and raced out, I was told by the patron's friend, who I'll call Ralph because he asked anonymity. A police report was not filed. "What's the use?" he thought.

Also not reported was an assault on a female co-worker of Ralph's sister at 13th and Spruce.

At Bliss, Ralph chased the teen but couldn't catch him. Some black passers-by who had no role in the attack laughed and taunted him, saying, "Look at the white boy, he's scared," Ralph said.

Some people may not want to hear this, but just as some white people don't like black people, some blacks don't like whites. But white people don't rampage in black neighborhoods.

They're afraid to.

Before this gets too black-on-white, the day before the flash mob, Belinda Moore, an African-American woman, was pummeled in Southwest Philly by a wolf pack of 20 black kids ranging from 9 to 15 playing "Catchin' Rep." Any given day, on black radio you can hear black people say that some in their community are afraid of their own young.

But behavior is more a function of class than race. Anti-social behavior is associated with bad education and poor parenting, and that's prevalent in poverty zones, which in Philadelphia are minority neighborhoods.

Fein is "dismayed" that attacks are occurring with more frequency, adding that many of the attackers "live in an environment where violence is part of the daily life." His Philadelphia Collaborative Violence Prevention Center works "diligently to reduce that violence in their neighborhoods and communities," he said.

Fein has his work cut out for him.

E-mail stubyko@phillynews.com or call 215-854-5977. For recent columns: