Packs of young people rampaged through Hollywood, knocked down people and stole cellphones and other items before a police sweep halted the marauding and arrested a dozen people, police said Wednesday.
Calls began coming in at around 8.30pm (local time) Tuesday that youths - possibly organised through social media - were robbing people and stealing T-shirts and other goods from businesses, mainly on Hollywood Boulevard, Lt Ray Valois said.
Fifteen to 20 young men and women were "attacking victims and taking property," he said. "Some people may have been knocked down or punched."
No serious injuries were reported.
As many as 40 people may have been involved, splitting into smaller groups and reforming as they looted people nearby, Valois said.
Cmdr Andrew Smith said it was believed that the youths took advantage of a redeployment of police to Los Angeles' Crenshaw District, where violence had broken out the previous night over the acquittal of a Florida neighbourhood watchman, George Zimmerman, in the shooting death of an unarmed black teenager, Trayvon Martin.
But was unclear whether the Hollywood marauders were inspired by either the Crenshaw violence or anger at the Zimmerman verdict.
Fourteen people were arrested Monday night in the Crenshaw District after about 150 people ran through the streets, jumped on cars, tried to break store windows and punched bystanders. Two protests Tuesday night in Crenshaw and downtown were peaceful.
Surveillance video aired on television showed youths roaming along the famed Hollywood Walk of Fame.
"None of these individuals was protesting, there was no physical evidence of that," Valois said. "They may be acting out."
Several people told journalists that they were surrounded by groups of youths that forced them to hand over cellphones, a watch and other items.
"I was filming celebrities coming out and all of a sudden I was approached by this group of people and one got my phone," Lori Aceves told KNBC-TV.
"And all I know is this lady next to me - I have no idea who she was - but she grabbed me by the arm and she tucked me in and she moved me to a different store."
Her iPhone and other stolen items were later found in a backpack, the station said.
More than 100 police officers were called in and most arrests were made within five minutes, but officers swept the area for about an hour before the operation was completed, Valois said.
Twelve people - all but one under the age of 18 - were arrested and none lived in Hollywood, he said. Eleven were held on suspicion of robbery and one for receiving stolen property.
It appeared that the attackers were loosely organised through social media, including some Web messages urging people to gather in Hollywood "to riot," Valois said.
"They said, 'let's go to Hollywood and have a riot,"' Smith said, referring to a specific group of kids who communicated on social media to organize the disturbance.
Other police officials said investigators had not yet determined the overall role social media had played in the disturbance.
It appeared that those arrested knew each other and while they may have arranged the rampage "it was not an open call," officer Bruce Borihanh said.
"It's more of an individual group action rather than any kind of social flash mob," he said.
The area may have been chosen because "Hollywood's open late, there's a lot of people here, we get a lot of media attention here," Valois said.
"I would not allow this to discourage any tourists or anyone coming to Hollywood Boulevard," he said. "I still believe it's a safe place."