A flash mob of up to 40 teenagers has rampaged through Hollywood, knocking down tourists and stealing phones and other items before police moved in to arrest them.

Police said Hollywood Boulevard was targeted on Wednesday by the gang, who had organised themselves "to riot" through social media.

Emergency calls reported that youths were robbing people and stealing t-shirts and other goods from businesses, mainly on Hollywood Boulevard, said police lieutenant, Ray Valois.

Up to 40 people may have been involved, with the gang splitting into smaller groups and reforming as they looted people nearby, 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.

Police commander 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.

Several people said they were surrounded by groups of youths, who forced them to hand over telephones, watches 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 a local television station. "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 telephone and other stolen items were later found in a backpack, the station said.

It appeared that the attackers were loosely organised through social media, including some internet messages urging people to gather in Hollywood ``to riot,'' Valois said.

Smith said members of the group communicated on social media to organise the disturbance. "They said, ‘let's go to Hollywood and have a riot’.”

Other police officials said investigators had not yet determined the overall role social media had played in the disturbance.