It was a warm day in the middle of a long holiday break, and trouble was brewing in Northeast Philadelphia.

As many as 400 teenagers converged Tuesday at Philadelphia Mills Mall, alerted via social media to gather. Security officers tried to block them from entering, but dozens eventually made their way inside. Four teens were arrested after assaulting police and generally creating a disturbance that took two hours to clear.

That the online app Snapchat enabled teens to quickly organize a ruckus at the mall for a second consecutive day was "a challenge," said Lt. John Stanford, a Philadelphia police spokesman. "It allows them to have access to more people, and for more of them to communicate on a much larger scale."

But police don't blame Snapchat, Instagram, Twitter, or Facebook, all of which have been used to organize flash mobs in the city and across the country. Youths need to be responsible for their behavior, but they are not the only ones, Stanford said.

"Parents have to be part of the process," said Stanford. "Hopefully, parents will see these reports and take a look at what their kids are involved in."

Philadelphia police and mall security continued to brace Wednesday for more trouble. In South Jersey, Cherry Hill Police Chief William Monaghan stationed extra marked and unmarked patrols at Cherry Hill Mall, in part because of the flash mob activity in Philadelphia and elsewhere.

"Any type of unruly behavior is not going to be tolerated," Monaghan said. "The mall and our shopping here in town makes us a destination. A lot of people are off, and they want to enjoy the holiday season, and unfortunately, it comes to this."

Monaghan also expressed frustration that youths often are left to their own devices for long periods.

"The parents need to understand: You can't just drop your kid off at the mall for eight hours and let them run around and let them hang out," Monaghan said. "It's not a playground. A lack of supervision at times can cause a degrading of how kids behave, and then issues arise."

At Philadelphia Mills, formerly Franklin Mills, teens congregated Tuesday in the food court, where some began fighting. When they were ordered to leave, the youths resisted, and the chaos escalated.

One teen pushed police. Another tried to wrestle an officer.

" 'Why don't you take that uniform off and fight me? . . . I will punch you in the face. . . . Karma is a bitch and you are going to get yours,' " police reported one of the teens screaming after refusing to leave the food court.

Another juvenile threatened to shoot officers, police said. Outside, some teens kicked cars in the parking lot.

No one was injured, but mall patrons were shaken, Stanford said.

"Kids will be kids - we've all been there - but there's a big difference between being mischievous and possible criminal behavior," Stanford said. "As a parent, the last call you want to get at work is that you need to come and get your kid at the police station."

Police recognize the students' right to enjoy their holiday break, Stanford said, but the department will coordinate with mall authorities and SEPTA - most teens arrived at the mall via bus - to try to prevent problems.

"Our officers showed a tremendous amount of restraint," Stanford said. "We just want to disperse them and send them on their way. We're not looking to put kids into a school-to-prison pipeline."

The Philadelphia Mills incidents Monday and Tuesday were part of a string of such problems across the country this week.

On Monday, a mall in Aurora, Colo., was closed after 500 people - mostly youths - gathered there when a fight broke out. Fifty police officers were required to quiet a crowd, and five juveniles were arrested.

That incident was triggered by a social media post, police in Aurora said.

Other mall problems were reported in Ohio, Connecticut, and elsewhere.

Chris McGoey, a Los Angeles security consultant and an expert on mall security, said this is a prime week for trouble.

"You've got a bunch of bored teenagers, and malls are a natural, warm, gathering-hangout place," McGoey said. "It's the holiday season, they've got nothing to do, and there's no school."

Flash mobs are tough to control, even when authorities are tipped off, as they were in Philadelphia, McGoey said.

"When all those people show up, a mall cannot handle that," he said. "Even the police get overwhelmed."

With the evolution of technology and teens' appetite for quick gratification on social media, mall management should get ready now for next holiday season, McGoey said.

"Next year," he said, "it will probably be more sophisticated. They should start planning now."