LOS ANGELES (AP) -- Protesters ran through Los Angeles streets Monday night, breaking windows, attacking people on sidewalks and raiding a Wal-Mart store, while others blocked a major freeway in the San Francisco Bay area in the third night of demonstrations in California over George Zimmerman's Florida acquittal in the killing of Trayvon Martin.

Fourteen people were arrested after multiple acts of vandalism and several assaults in Los Angeles' Crenshaw District. One man was seen in the street with a head injury and a TV crew received minor injuries in an assault.

More than 300 officers were called to the scene and were at first slow to directly engage protesters in an attempt to allow a peaceful end to the demonstration, police Chief Charlie Beck told a press conference. But the chief said police would take a much stricter posture in the coming nights.

"This will not be allowed to continue," Beck said.

Los Angeles Mayor Eric Garcetti said the trouble was caused by a small group exploiting the situation.

"The trial that we saw in Florida has ignited passions but we have to make sure that it will not ignite the city," Garcetti said.

Several hundred mostly peaceful protesters had gathered at Leimert Park southwest of downtown, many of them chanting, praying and singing.

But a smaller group of between 100 and 150 people splintered off, blocked traffic on nearby Crenshaw Boulevard, jumped on cars and broke windows at liquor stores and fast food outlets.

Several protesters ran into a Wal-Mart store and knocked down displays before store security chased them out. Police began guarding the door.

Tonya Williams was shopping with her daughter when the protesters burst in and security briefly locked down the store.

"We thought we were going to be stuck in there," Williams said. "We saw the merchandise all thrown around. They had pulled the rack down, and there was merchandise all over the floor."

TV news helicopters showed some people kicking and punching others along the street, including two people sitting on a bus bench.

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Police declared an unlawful assembly about three hours after it began, and most of the crowd left the street.

Among those assaulted were a reporter and photographer for Los Angeles TV stations KCBS and KCAL. Reporter Dave Bryan and the photographer were interviewing a man in Crenshaw when a man tackled them and ran off. KCBS-KCAL reported (http://cbsloc.al/15BU9C3 ) the crew was treated for minor injuries.

Garcetti, who returned early from an East Coast trip because of the demonstrations, praised the "overwhelming majority" who protested peacefully.

"We are a better city than what we have seen tonight in the hands of a few people," the mayor said.

"The Martin family didn't ask anybody to break car windows," he added. "They didn't ask anybody to take little kids' scooters. They didn't ask anybody to attack businesses and they certainly didn't say to take over traffic in the streets."

In Oakland, dozens of demonstrators briefly blocked Interstate 880 at the end of rush hour before lanes were cleared by authorities. Several protesters laid their bicycles on the ground in front of stopped cars.

"You've got to go. You will go to jail," one police officer shouted at demonstrators who were blocking traffic, the Oakland Tribune reported. However, police decided not to make arrests as the marchers, chanting "Justice for Trayvon Martin," were directed back to surface streets.

Later, another group tried to march up the onramp to Interstate 580 before being turned away by Oakland police and California Highway Patrol officers.

The freeway protesters broke off from a larger group organized via social media that gathered at Oakland City Hall about an hour earlier.

Police made nine arrests for crimes including assault with a deadly weapon and vandalism, police spokeswoman Johnna Watson said.

A waiter guarding windows at a restaurant was hit in the face with a hammer, and protesters threw fireworks at police in riot gear, newspapers reported. Police deployed flash-bang grenades.

The waiter's co-workers helped him ice his head as they waited for an ambulance, the Oakland Tribune reported (http://bit.ly/18i9j1E ).

Witnesses told the newspaper that a protester was also injured after being struck by a police projectile. Police did not immediately respond to a request to confirm that report.

Demonstrations began in California Saturday night after the Florida jury returned its verdict.