Philadelphia police leaders admitted that they were caught off-guard last weekend when a huge gathering of teenagers descended on South Street and created havoc along one of the city's premier destinations.

But, at a community meeting in Bella Vista yesterday, they vowed it wouldn't happen again.

City and state police will have a huge presence on South Street and in the surrounding neighborhoods tomorrow night, enforcing curfews for juveniles and taking a zero-tolerance approach to mischief.

"This process will continue until we get this back to what we feel is a comfort level," said Deputy Commissioner Kevin Bethel. "There was no way we could have anticipated what occurred."

Police believe that teenagers from throughout the region have been using online social-networking sites, such as MySpace, to plan to gather in huge groups. Bethel would not discuss whether or how police are monitoring those sites.

Teens have gathered this way along South Street on Saturday nights for several weekends - and police have seen this sort of thing this year in University City and Upper Darby - but nothing on a scale of what happened last weekend.

"We recognize the dynamics of South Street have changed and could change again," Bethel said.

Estimates of the number of kids who congregated Saturday have varied, going as high as 10,000.

In the worst of the resulting behavior, a cabdriver was carjacked at 12th and South Streets, and a woman and her passenger were pulled from their cars and assaulted. Teens moved along to South Broad Street, where a convenience store was vandalized.

Several teens were charged with violating the midnight curfew, but no one was arrested in the more violent episodes.

"These kids came down here with one intention, and that's to be as disruptive as they could possibly be," Al Grafstrom, owner of La Fourno Ristorante on South Street, told police yesterday. "I'm glad to hear that your message is zero tolerance."

Last weekend, he said he saw a teen kick at an officer riding past on a bicycle late in the evening. The officer's reaction as he attempted to apprehend the youth created a shock wave among the teens.

"Without exaggeration, it was hundreds of kids that just stampeded from Sixth and South to Eighth and South," he said.

Bethel said police would have a staging area at Broad Street and Washington Avenue and would monitor public transportation for large groups of teens coming into the area.

Bethel estimated the crowd last Saturday was mostly made up of youths between the ages of 11 and 17, from both the city and the surrounding counties.

"In all likelihood, this started out as something innocent," he said. "Only a small number want to agitate and create this kind of situation."

He would not say how many officers would be on hand tomorrow, or how they would be deployed, but on a normal Saturday night the South Street police substation would have 28 officers.

Lt. Gerry McShea, who commands the substation, said police would not be an occupying force - to the delight of residents and business owners who depend on visitors to South Street.

"It's a delicate balance out there," he said. "We don't want to send the message this isn't a place to be free."

South Street long has been a place where loud music, public drunkenness, and the occasional waft of marijuana smoke have congealed to form the area's funky vibe and reputation.

Although some might argue the strip has gone slightly upscale in recent years - and lost some of its harder edges - residents say the nuisance crimes have remained.

Larry Levin, who lives at 10th and Fitzwater Streets, said police aren't as proactive about minor offenses in the colder months, which he feels gives South Street patrons the impression they can "act like idiots" in the busier, warmer months.

"Just enforce it all year round," he said. "That's all we ask."

Several of the dozens of residents who attended yesterday's meeting stressed that larger issues were at play in the recent mayhem - bad parenting and a lack of activities for teenagers among them.

"We know we have a number of issues . . . the social issues and the like," Bethel told them. "Our purpose is to create a safe environment for you."

Laura Blau, an architect who lives on South Seventh Street and has an office nearby on Passyunk Avenue, said she was pleased with the measured response pledged by police.

"These kids use bad judgment. You don't teach good judgment by giving them a criminal record," she said. "I have a sense they have a balanced approach. They realize the complexity."

Steve Schatz has lived along South Street for about 20 years and watched its evolution. He said he was confident that this latest challenge would pass.

"This is something that's a blip," he said. "The street will return to normal and it will be a friendly place."