iPhones sit on the counter of a damaged Apple Store in Philadelphia on Wednesday. AP

Groups of teenagers swarmed into stores in several areas of Philadelphia in an apparently coordinated effort, stuffing plastic bags with merchandise and fleeing, authorities said. Police arrested 15 to 20 people.

The flash mob-style ransacking on Tuesday night at stores including Foot Locker, Lululemon and Apple came after a peaceful protest over a judge’s decision to dismiss murder and other charges against a Philadelphia police officer who shot and killed a driver, Eddie Irizarry, through a rolled-up window.

Those doing the ransacking were not affiliated with the protest, Interim Police Commissioner John Stanford said at a news conference, calling the group "a bunch of criminal opportunists.”

Video on social media showed masked people in hoodies running out of Lululemon with merchandise and police officers grabbing several and tackling them to the sidewalk.

Photos of a sporting goods store at a mall showed mannequins and sneakers scattered on the sidewalk.

The thefts and unrest stretched from downtown to Northeast and West Philadelphia, leaving smashed display windows and broken storefront coverings.

"This destructive and illegal behaviour cannot and will not be tolerated in our city,” Mayor Jim Kenney, a Democrat, said on social media, calling it a "sickening display of opportunistic criminal activity.”

His administration is working with police to assess "which areas of the city may need increased coverage or additional resources,” he said.

People appeared to have organised efforts on social media, Stanford said.

Police are investigating "that there was possibly a caravan of a number of different vehicles that were going from location to location" and that a couple of people in the caravan were in custody.

There was a high number of young people in Philadelphia's downtown business corridor, called Center City, shortly before 8:00 pm, and some officers stopped a group of males "dressed in black attire and wearing masks," according to a police news release.

At that time, reports and 911 calls came in about the Foot Locker store. When police arrived, they found it had been "ransacked in a coordinated attack,” the news release said. The juveniles fled, and at least one adult as arrested.

By 8:12 pm, police responded to similar calls at Lululemon, where people were arrested. Shortly afterward, calls directed police to the Apple Store. No arrests were made there, but the store lost phones and tablets, although many have been recovered, the news release said.

No injuries were immediately reported, but CBS Philadelphia reported that a security guard was assaulted at the Foot Locker.

In North Philadelphia, a pharmacy owner said his store was broken into for a third time and items taken.

"I understand, you know, desperate times for people call for desperate measures and there are plenty of opportunists that just see drugs," Benjamin Nachum told CBS. He said, though, that a safe in his store did its job, "so there won't be any illicit narcotics hitting the street.”

The thefts occurred the same day Target announced it will close nine stores in four states, including one in New York City’s East Harlem neighborhood, and three in the San Francisco Bay Area, saying that theft and organized retail crime have threatened the safety of its workers and customers.

Associated Press