The police say high school kids, linked by social media and cell phones, came from all over the city.
The mob reached critical mass at The Gallery Mall and exploded.
The Gallery is ground zero because it's a public transit hub and the kids travel with their SEPTA school passes.
At just before 5:00 p.m. Tuesday, upwards of 150 kids were ejected from the mall by security.
Police say they then went on a rampage down Market Street toward City Hall. Some went into the Macy's department store and vandalized display cases.
"Our business community is going through a very trying time because of the weather, and the last thing we need is for these kids to come down here and create an environment where no one feels safe and comfortable to shop," said Dep. Comm. Kevin Bethel of the Philadelphia Police.
The police held a flurry of meetings with the business community and school officials on Wednesday, determined to head off further trouble.
The city has seen violent flash mobs before. Last May hundreds of young people rioted on South Street and then marauded up Broad where they trashed a convenience store.
This time police arrested 16 teenagers, and they are making a tough stand this time.
In other words, the kids won't be getting off with a slap on the wrist.
"We will not tolerate it. You will be locked up, you will be going into the court system," Bethel said.
Police say they'll be working with the schools to have the kids expelled.
The school district says it is working with the police and already has truancy patrols in The Gallery.
Action News found high schoolers there at lunchtime who said they had early dismissal. They also say they were kicked out of the food court.
"They made us leave, and we were just sitting there waiting for our food and they made us leave," said 12th grade student Shanay Harrell.
"He told us we had to leave from the table," said 12 grade student Tearah Yuille. When asked why, she thinks, she was made to leave, she replied "I guess we're not supposed to be here at a certain time because we are in high school."
Security is tight in the Gallery and on the streets. The police have deployed extra plain clothes and uniformed officers and they're keeping an eye on various social media sites for signs of trouble.
