In this photo taken July 14, 2011 actors dressed in beach attire participate in a flash mob as part of a commercial for McDonald's in Chicago ((AP Photo/Charles Rex Arbogast)
Flash Mobs: The July 4 fireworks display in the Cleveland suburb of Shaker Heights was anything but a family affair.
As many as 1,000 teenagers, mobilized through social networking sites, turned out and soon started fighting and disrupting the event.
Thanks to social networks 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, recent rioting and looting has been 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.
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