LA Times:

In the last several days, there have been several reports of such group crime waves in South L.A., Hollywood, San Bernardino and Victorville. Long Beach police are bracing for another one Friday.

These so-called bash mobs of "flash mob" crime waves are organized through social media and have been a problem in Chicago, Philadelphia and Washington. In April, 28 Chicago youths were arrested on suspicion of attacking pedestrians along the city's famed Magnificent Mile. Illinois Gov. Pat Quinn signed legislation in May enacting stiffer penalties for people who text or use social media to organize mob attacks.

Long Beach police warned in a statement that participants could face severe penalties.

"The mere participation in such an event can result in felony charges including conspiracy, and are punishable by imprisonment in the state prison," according to the statement.

Police said they feared bash mob organizers planned to hit Long Beach at 2 p.m.

Long Beach experienced such a gathering July 9, when more than 100 people descended on stretches of downtown in an organized, sudden crime rampage.

On Monday, a group of unruly young people broke off from hundreds gathered for a Trayvon Martin prayer vigil and rushed into a Wal-Mart on Crenshaw Boulevard, where they tossed merchandise and tried to break into a jewelry display case.

In Hollywood on Tuesday night, a flash mob of thieves rushed down Hollywood Boulevard, stealing phones, knocking over tourists and vandalizing shops, according to police, who said it may have been related to the George Zimmerman verdict. Twelve people -- 11 juveniles and one 18-year-old -- were arrested on suspicion of robbery.

On Wednesday night in Victorville, authorities arrested 17 people after a group allegedly tried to force its way into the Mall of Victor Valley.