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Flesh wound sustained by an officer during the Black Friday melee.

Of the 79 people arrested at last Friday's melee in Union Square, only 30 percent were San Francisco residents, according to stats released by today by the SFPD. Fifty were from elsewhere in California, and 6 had come in from out of state.That lends credence to the theory that people commute to protest in downtown San Francisco, just as drug dealers have been known to BART in from outlying areas to peddle their wares in the Tenderloin. Police Chief Greg Suhr suspects that a loose army of interlopers are embedding themselves in what would otherwise be peaceful demonstrations.He mentioned one splinter group in particular, but didn't want to "flatter them by saying their name."The Union Square protests, which coincided with a ballsy demonstration at West Oakland BART, in which 14 people chained themselves to a train, turned violent shortly after a group of protesters interrupted the annual tree lighting ceremony at Macy's. Demonstrators threw bricks and bottles at the police, smashed store windows, and looted a Radioshack. Five officers sustained injuries, including one gnarly face gash from a flying bottle. In the end, four people were booked on charges ranging from assault of an officer, to possession of stolen property, to having prior warrants. Police are still canvassing the area for video footage that could be used to make more arrests — Union Square has at least 300 surveillance cameras arrayed throughout the area, making it one of the most closely monitored places in San Francisco. (Granted, those cameras failed to solve a murder in the downtown plaza last year.)Suhr is optimistic that his department will make as many arrests as possible. In the meantime, Christmas season proceeds apace in the square: On Friday, the Macy's tree was lit with 33,000 LED lights