Long Beach police brace for a possible "bash mob."

Long Beach police are bracing for a possible "bash mob" Friday and warning potential participants that they will be arrested if they contribute to mayhem and commit any crimes.

A Long Beach Police Department news release about the threatened event defined a bash mob as a "planned, sudden assembly of individuals who attack innocent people and businesses" and then quickly disperse after committing the crimes.

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

The news release said that participants may not know how severe the penalties could be and advised that "the mere participation in such an event can result in felony charges including conspiracy, and are punishable by imprisonment in the state prison."

Police said they expect Friday's mob to gather at 2 p.m. and to involve many of the same people who participated in the July 9 rampage.

A somewhat similar crime event broke out Tuesday night in Hollywood that Los Angeles police believe was organized via social media to take advantage of the LAPD's redeployment of officers to protests and marches in other parts of the city in the wake of George Zimmerman's acquittal in the shooting death of Trayvon Martin.

"Flash mob" crime rampages organized through social media have been a problem in other cities, including 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.

Until now, flash-mob crime has not been a major problem in Los Angeles, LAPD officials said.

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ari.bloomekatz@latimes.com