No doubt, some of the teenagers who in recent weekends showed up in Philadelphia after getting the word via social-networking sites were intent on mischief or even crime - but, police officials say, they were not part of a wide-scale plot.

Furthermore, despite some of the violence that resulted once thousands of teens gathered along South Street last Saturday night, there does not appear to be anyone inciting trouble online, or any ringleaders of the massive gatherings.

"We don't think this is one big group," said Lt. Frank Vanore, a Philadelphia police spokesman. "I'm sure these people don't all know each other. I just think it's a chain reaction."

He said the only people who would face charges were those caught on the street "in a criminal act."

Several times this year, police have seen hundreds of teens suddenly gathering in one place. It's happened in Upper Darby, University City, and several weekends in a row on South Street.

But last Saturday night, as many as 10,000 youths hit South Street, catching police off-guard and overwhelming officers. A cabdriver was carjacked and two other people were pulled from their cars and assaulted.

"What happened in Philadelphia last weekend, they're lucky somebody didn't die," said Upper Darby Police Superintendent Michael Chitwood. "It's like going to a 76ers game, 8,000 to 10,000 people."

Philadelphia police have pledged to have a huge presence on South Street tonight, enforcing the midnight curfew for juveniles and taking a zero-tolerance approach to disorder along South Street, which has long been one of the city's most mischievous corridors.

Police would not say whether they were monitoring online social networks such as MySpace.

Chitwood said there was no criminal activity when teens congregated in Upper Darby, but "if I can identify someone who's inciting violence, I'd do everything in my power to charge them."

Finding someone stirring up trouble online could be difficult because many sites, such as Facebook, allow users to restrict those who may view postings and other information.

The Philadelphia Daily News quoted someone using the name Malcolm Jamal Smith who this week posted on OurSpace, a site that caters to African Americans, as saying he was "organizing some south street riots."

Such a person was not listed on OurSpace yesterday.

"Obviously, you'd have to look at the whole post," Chitwood said. "What are the words used? Do they in fact incite a crowd? Does something happen?"

Vanore also said that if someone posted an ominous message like that, there would be no way to know if they were serious or perhaps mocking the hype around the gatherings.

"That's what happens when something gets this much attention," he said.

Plus, Chitwood said, "they could be e-mailing from Russia for all we know."

Philadelphia police have reached out to city schools and conducted some public service announcements this week to get word out to parents about the gatherings.

Deputy Commissioner Kevin Bethel said this week that he believed the gatherings "started out as something innocent," and many teens might have joined in with the blessing of their parents.

Police hope that parents now have gotten the message to keep their children away from the gatherings.

"Know where your kids are going," Vanore said. "Some of these kids, you're talking about 12, 13 years old."

Even if crimes are not committed online, police know they can't ignore the Internet.

"My experience was always knock and talk. . . . You've got to hit the streets," Chitwood said. "Now you've got to hit cyberspace."