Philadelphia police said a "flash mob" with thousands of teenagers and young adults swarmed South Street Saturday night causing businesses to close early and bringing law enforcement from across the city to control the crowd.

Lt. Ron Ball said what he was characterizing as a flash mob started around 9 p.m. He said it was similar to what had occurred recently in Center City. "Someone starts Twittering," he said.

Police were alerted to the gathering by parents who called Philadelphia police after seeing messages on their childrens' mobile devices telling them to come to South Street.

At about 11 p.m., as a group of about 50 teens approached 10th and South, trying to head back east, Ball called for assistance. "You need to get me five or six more officers," he said, "so we don't lose the ground we just gained."

One youth who declined to give his name explained why he was on South Street.

"Some people call," he said. "They tell you to come down, so you come down."

Sgt. Ray Evers said early Sunday morning police had made three arrests - two for disorderly conduct and one for aggravated assault. Evers said there were no reports of injuries or damage.

Police had spent several hours trying to disperse the huge crowd and move the flood of teens west toward Broad Street.

Shortly before 11 p.m., Officer Christine O'Brien, a spokeswoman for the Police Department, said additional officers were being rushed to the South Street area, including members of the Narcotics Strike Force.

The crowd brought traffic along South Street to a standstill and caused drivers to nervously sit in their cars while police tried to disperse the youths.

Businesses along South Street locked their doors, and some restaurants kept their patrons inside to keep them safe.

"This is the worst I've ever seen it," said Barbara Bender, who has been a waitress and manager at South Street Souvlaki at 509 South St. for about 30 years.

Normally the popular restaurant is open Saturdays until 10:30 or 11 p.m., but it closed last night at 9:30.

"It's a bunch of young kids that are creating havoc, and it's killing our business," she said. "It's scary."

Bender observed some fights, a few outbreaks and then masses of teens running madly in one direction before shifting and running another way.

"The cops are really trying," she said. "But it's overwhelming."

Police Capt. Michael Ryan said shots were fired near 13th and South Streets about 11:30 p.m. There were no reports of injuries.

He said prior to the shots being fired one man was arrested and a weapon was taken from the suspect.

Along South Street east of Broad Steet there was a significant police presence by 11 p.m. and by midnight it appeared that the vast majority of the crowds had been moved off South Street.

Several people were injured when a flash mob of at least 100 converged at Broad and South Streets on May 30. A 54-year-old man riding his bicycle home from work was critically injured when he beaten by a group of young men. A cabdriver was assaulted and robbed at 12th and South. And two people were pulled from a vehicle on Broad Street and assaulted and robbed.

Mobs of teens have disrupted Center City three times in the last three months. On Dec. 18, students from several city high schools responded to a call via Facebook to participate in a massive gathering in response to fights at the Gallery food court. Some of the teens later randomly attacked pedestrians on nearby Center City streets.

In a Feb. 16 melee, 150 teens spilled out of the Gallery during rush hour and then rampaged through the nearby Macy's, knocking down pedestrians and damaging displays.

After fights broke out in the late afternoon of March 3, police made 28 arrests and charged the teens with felony rioting.

Trials are scheduled for March 22 and 23.

Contact Inquirer staff writer Martha Woodall at 215-854-2789 or martha.woodall@phillynews.com