Police made a show of force in Center City today after they were alerted that large groups of teens planned to gather in response to two earlier fights at the Gallery.

Throughout the afternoon, police monitored gangs of youths - sometimes numbering more than 100 - as they roved around Center City.

Some bystanders were assaulted by one of the gangs.

Police Lt. Michael Brady said there were fights yesterday and Wednesday at the Gallery's Food Court that resulted in four arrests.

Calls for a massive gathering then surfaced on Facebook, he said.

He said students from at least three high schools, including Overbrook and Kensington, were involved.

"They're from other sides of the city. We don't know why they're doing it, but they're gathering here," he said. "They're trying to create some sort of disruption."

"One of the groups is smacking people," he said. "When anybody confronts them they become combative."

Fernando Gallard, a spokesman for the Philadelphia School District, said officials there heard from city police "regarding possible altercations involving students in Center City, specifically at the Gallery."

Gallard said police informed them the students came from Audenreid and South Philadelphia High Schools and from Preparatory Charter School of Math, Science Technology and Careers, which is located in Point Breeze.

Security guards were posted at every entrance to the Gallery and did not permit juveniles without adults to enter the mall, said police spokesman Lt. Frank Vanore.

Large groups collected outside the doors and blocked the entryways, Vanore said. Police arrested two teens when they refused to disburse.

Officers followed groups of teens on Market, Chestnut and Samson Streets from 9th to 16th Streets.

Jane Lessner, a Center City lawyer, said she just left Macy's after doing some holiday shopping when she saw a group of 70 to 100 youths approaching.

She was trying to make her way through the group when "they started pushing me."

"One pushed me into another, who pushed me into another, who pushed me into another until one girl hauled off with her fist and hit me in the face," Lessner said.

The punch knocked out a lens of her glasses and left here with a bloodshot eye.

Brendan Meehan, 44, of Media, who also was Christmas shopping, saw the attack and called 911.

"I saw a teenage girl hit her right in the face," he said. "The girl came back and taunted her as I called 911."

Lessner, he said, "was shaking and held onto me as I called."

Police took her to Hahnemann Hospital for treatment.

Another witness said he saw the same gang knock down a bicylist.

At one point, six police vehicles responded to a Wendy's on 15th and Chestnut when a call came that there were 100 teens inside. The officers dispersed the group.

In the midst of the confusion, a man robbed a bank at 18th and Market Streets about 2 p.m.

Gallard, the school district spokesman, said that the district was working closely with Philadelphia police and had dispatched extra school police to the involved schools.

"We had a very smooth closing in our schools," Gallard said.