As many as 1,000 teenagers, mobilized through social networking sites, turned out and soon started fighting and disrupting the event.
Thanks to websites like Twitter and Facebook, more and more so-called flash mobs are materializing across the globe, leaving police scrambling to keep tabs on the spontaneous assemblies.
"They're gathering with an intent behind it -- not just to enjoy the event," Shaker Heights Police Chief D. Scott Lee said. "All too often, some of the intent is malicious."
Flash mobs started off in 2003 as peaceful and often humorous acts of public performance, such as mass dance routines or street pillow fights. But in recent years, the term has taken a darker twist as criminals exploit the anonymity of crowds, using social networking to coordinate everything from robberies to fights to general chaos.
In London, rioting and looting over the weekend was blamed in part on groups of youths using Twitter, mobile phone text messages and instant messaging on BlackBerry to organize and keep a step ahead of police.
And Sunday in Philadelphia, Mayor Michael Nutter condemned the behavior of teenagers involved in flash mobs that have left several people injured in recent weeks.
"What is making this unique today is the social media aspect," said Everett Gillison, Philadelphia's deputy mayor for public safety. "They can communicate and congregate at a moment's notice. That can overwhelm any municipality."
A Philadelphia man was assaulted by a group of about 30 people who were believed to have gotten together through Twitter. In 2009, crowds swelled along the trendy South Street shopping district and assaulted several people.
On June 23, a couple dozen youths arrived via subway in Upper Darby, outside Philadelphia, and looted several hundred dollars of sneakers, socks and wrist watches from a Sears store. Their haul wasn't especially impressive but the sheer size of the group and the speed of the roughly five-minute operation made them all but impossible to stop.
"The good thing is there were no weapons and nobody tried to stop them, either," Upper Darby Police Chief Michael Chitwood said. "The only people that tried to stop them were the police when they rounded them up."
Dubbed "flash mob robberies," the thefts are bedeviling both police and retailers, who say some of the heists were orchestrated or at least boasted about afterward on social networking sites.
In recognition of the problem, the National Retail Federation issued a report last week recommending steps stores can take to ward off the robberies. There have even been legislative efforts to criminalize flash mobs.
The Cleveland City Council passed a bill to make it illegal to use social media to organize a violent and disorderly flash mob, though the mayor vetoed the measure after the ACLU of Ohio promised it would be unconstitutional. The bill was at least partly inspired by the Shaker Heights disturbances on July 4.
Jonathan Taplin, director of the innovation lab at the University of Southern California's Annenberg School for Communication, said he was not surprised to see people using social media for organizing flash mob robberies.
"You are essentially having a world where you have 25 million people who are underemployed and 2 percent of the population doing better than they ever have," Taplin said. "Why wouldn't that lead to some sort of social unrest? Why wouldn't people use the latest technologies to effect that?"
In Los Angeles last month, thousands of ravers forced rush-hour street closures when they descended on a Hollywood cinema after a DJ tweeted he was holding a free block party. The sudden crowd dispersed only after police fired bean-bag bullets at the restive revelers and arrested three.
And in April, a man was shot when hundreds of rival gang members congregated along the Los Angeles seafront in Venice, sparking pandemonium as people scattered for cover. The group had gathered after some of them posted on Twitter and police were still strategizing their response to the huge crowd when shots rang out.
Los Angeles police Capt. Jon Peters said law enforcement's challenge is to try to sift the ocean of tweets and Facebook updates for signs of trouble.
"We need to be able to get better on the intelligence side to pick up on communications that are going on," he said.
Gillison, the deputy mayor from Philadelphia, said the police department there has reached out to younger community members and friended some of them on Facebook. This enables officers to monitor the traffic that could generate flash mobs and some have been prevented, he said.
In April, about 20 teenagers entered G-Star Raw, a high-end men's clothing store in the Dupont Circle neighborhood of the District of Columbia, and stole about $20,000 worth of merchandise despite employees' efforts to grab the apparel back, store manager Greg Lennon said. D.C. police have investigated leads but have not made arrests in the case.
Lennon said he later saw Twitter postings, apparently written after the robbery, that referenced the theft, with one person describing having been in the store and making plans to come back.
The National Retail Federation said 10 percent of 106 companies it surveyed reported being targeted in the last year by groups of thieves using flash mob tactics.
"Retailers are raising red flags about criminal flash mobs, which are wreaking havoc on their business, causing concerns about the safety of their customers and employees, and directly impacting their bottom line," the federation said in a report, which advises retailers to monitor social media networks and report planned heists to the police.
That's exactly what Lennon does. He says he checks his store's Facebook page to see who's visiting, and monitors Twitter for any reference to his store and its merchandise.
Gillison and others blame at least part of the problem on bad parenting.
"They're 12 years old and not around the corner from their home. Where's their parent?" said Chitwood, the Upper Darby police chief. "If they're out doing flash mob thefts when they're 12, what the hell are they going to be doing when they're 16?"
