Every so often, like a dormant volcano, South Street erupts. The second-to-last time was Mardi Gras 2001, police say. Unfortunately, the last time was last Saturday.

Thousands of teenagers descended on South Street and South Broad, indulging in acts of destruction, carjacking, looting.

The new tweet in civil disobedience was that the kids met up - though not in a good way - as a "flash mob" through online social networking sites such as Twitter and OurSpace. Groups previously had gathered in University City and Upper Darby, though not in this number, creating such havoc.

"It seems, once a year, kids get energized to come down here, though not in this force," said Fluffy Palmer, a longtime resident at a well-attended community meeting with police Thursday. "Then it gets into a war zone."

'Tis the season. "When the weather gets nice, they can come out and act like idiots," resident Larry Levin said. "I won't go there between April and October."

For a half-century, South Street has been the gathering night spot for teens from all over town, especially when the weather becomes warm. It's an urban boardwalk.

Instead of being adjacent to the sea, the street is ringed by lovingly tended rowhouses, among Philadelphia's priciest, while adjacent to the Broad Street Line.

South Street's also a microcosm of the city, a confluence of class and culture and race. Few commercial areas remain so democratic. Art galleries face adult stores. Destination bistros flourish along an artery-clogging cluster of pizza and cheesesteak joints.

There appears to be no lone intention, no exclusivity, no theme - aside from fat - to the array of businesses, unlike the Boho hipsterism of Northern Liberties or Port Fishington, or the insistent trendiness of Old City.

The result: South Street is younger and more racially diverse, at least when it comes to commerce.

It's also more prone to chaos.

Though there are plenty of bars, and on a sultry Sunday morning the place can smell as if it's been hosed down with lager, it's a destination for the pre-legal crowd.

"Talking about this age group, it doesn't take that much to get them motivated," said Deputy Police Commissioner Kevin Bethel in a mastery of understatement.

As one resident asked, "Where else can kids who can't drink go and hang out for little money?"

That's a question well worth asking. Schools close in a few weeks. In this economy, it's hard enough for parents to land jobs, let alone teenagers.

There are rec centers, pools will soon open, but that's not enough. Kids get bored. Then they get intensely stupid. And now, technology enables them to get stupid in a text-message moment.

The astonishing thing about last week's incident on South Street isn't that it happened, but that it doesn't happen more frequently.

Suggestions of how to deal with the problem ranged from high fines and more enforcement of quality-of-life crimes to having a court on the premises.

Perhaps one could be set up in Condom Kingdom.

"I'm not their parent. I don't want to be a social worker," said Al Grafstrom, owner of La Fourno restaurant. "I have my job, and that's to run my business."

The hard part is to give kids other places to go, more things to do, disperse a potential mob through diversion, activities, entertainment, and sports.

On any given Saturday, as many as 28 officers are deployed on South Street. Tonight, it may resemble a police state, complete with state troopers. For five years, Lt. Gerry McShea has commanded the South Street Mini Station. He knows a melee may happen again. There are July Fourth festivities coming up, followed by the Greek Picnic.

"We don't want to be an occupied neighborhood," McShea said. "We want people to come here."

Like that's ever been South Street's problem.

Contact staff writer Karen Heller at 215-854-2586 or kheller@phillynews.com.