Chief Charlie Beck and Mayor Eric Garcetti, who took office barely two weeks ago, said peaceful protests are welcome but violence won't be tolerated. Beck vowed that anyone committing violence will be arrested.
"Your actions ... will reduce the power of the message of this community and that is wrong, that is a shameful act," he said.
Los Angeles and Oakland have been flashpoints for violent reactions to Saturday's acquittal of Zimmerman in the death of Trayvon Martin.
In the Crenshaw District of Los Angeles, 14 people were arrested Monday night after about 150 people split from a peaceful demonstration at a park, running through the streets, jumping on cars, trying to break store windows and punching bystanders. A Wal-Mart store was vandalized.
A reporter and photographer for Los Angeles TV stations KCBS and KCAL also were assaulted and transported to a hospital with minor injuries. A police officer was punched in the chest on Sunday and a 24-year-old man, Brandon Bell, was charged Tuesday with battery and resisting arrest.
Deputy Chief Bob Green, who heads the LAPD's South Bureau, said officers were pelted with rocks as they stood by monitoring the protest. He blamed the incidents on "knuckleheads" from outside the area who are trying to provoke a confrontation with police.
In Oakland on Monday night, demonstrators left a gathering at City Hall before briefly blocking Interstate 880, trying to march onto Interstate 580, throwing fireworks and assaulting a restaurant waiter with a hammer.
Oakland police used flash-bang grenades and made nine arrests for crimes including assault with a deadly weapon and vandalism.
Los Angeles officials are very sensitive to images of unrest in a city where the 1965 Watts Riots resulted in 34 deaths and 1992 violence following the acquittal of four white officers in the beating of black motorist Rodney King left 55 people dead and thousands injured.
The LAPD has spent years building relationships in the black community and it's now working with African-American leaders to head off more serious violence. More than 300 police officers responded to the Crenshaw demonstration and they were intentionally slow to directly engage protesters to allow a peaceful end to the demonstration.
Earl Ofari Hutchinson, president of the Los Angeles Urban Policy Roundtable, a south Los Angeles-based community activist group, said the department's response demonstrated the progress that's been made.
"They didn't wade in with stun guns and billy clubs, shooting up the joint, which might have happened 20 years ago," Hutchinson said. "I think that's a sign they've learned a few things."
Los Angeles Police Commissioner John Mack, a former president of the Los Angeles Urban League, said the public needs to differentiate between the troublemakers and the peaceful demonstrators.
"It's important we don't get carried away and get so focused on the few, who in my opinion clearly were not a part of the organized group and had their own agenda," Mack said. "Quite frankly, I'm not so sure that all of them even cared about Trayvon Martin."
Associated Press writer Terry Collins contributed to this report. Tami Abdollah can be reached at http://www.twitter.com/latams
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