Los Angeles officials have appealed for calm after protests turned violent over the acquittal of a neighbourhood watchman who killed a black teenager.

Fourteen people were arrested amid vandalism and assaults in a south-west district of the Californian city.

The city's police chief said his officers would adopt a tougher approach to any further disturbances.

George Zimmerman, 29, was cleared on Saturday of unarmed 17-year-old Trayvon Martin's death in Florida last year.

Federal officials are now investigating potential civil rights charges against Mr Zimmerman, who said he opened fire in self-defence.

'Ignited passions'

Los Angeles police said about 150 people splintered off from a peaceful vigil in the Crenshaw area on Monday, some of them jumping on cars and breaking windows at fast-food outlets.

This will not be allowed to continue Charlie Beck, Police Chief

Several protesters vandalised a Walmart in the neighbourhood. Aerial broadcast news footage showed troublemakers kicking and punching people on the street.

Police, who brought 300 officers to the scene, declared an unlawful assembly, and most of the demonstrators then dispersed.

Los Angeles Mayor Eric Garcetti held a late-night news conference after cutting short a trip to the US East Coast because of the disturbances in his home city.

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

He was joined by Police Chief Charlie Beck, who said: "This will not be allowed to continue."

Meanwhile, in Oakland, California, protesters briefly shut a highway during rush hour on Monday evening.

Image caption A man throws a bin into a shop window in Oakland

Police made several arrests amid disturbances in the centre of the city.

Rocks and bottles were thrown at Oakland police officers, the San Francisco Chronicle reported.

The newspaper also reported that a waiter trying to shield windows at a restaurant was struck in the face with a hammer.

It was the third night of protests in the state of California over the Trayvon Martin case.

Peaceful demonstrations and vigils have been held in cities across the US, with more protests possible.

On Tuesday, civil rights leader and broadcaster Rev Al Sharpton discussed his plans for vigils and rallies in 100 cities this weekend to call for federal charges against Mr Zimmerman.

Jury 'initially split'

One of the jurors of the sequestered, all-female panel of six told CNN in an interview aired on Monday evening that the jury had initially been split.

The juror, known as B37, said that she and two others believed that Mr Zimmerman was not guilty, while the other three thought he was guilty.

She said that after much deliberation, they ultimately agreed that not guilty was the only verdict they could reach based on the law in the case.

B37 said she believes Mr Zimmerman's "heart was in the right place" and that the only thing he was guilty of was a lack of "good judgement" in confronting the teenager.

She added that while "both [George Zimmerman and Trayvon Martin] were responsible for the situation", she had "no doubt George feared for his life".

B37 initially said she would write a book on the trial, but has since changed her mind.

Mr Zimmerman's parents, meanwhile, told broadcaster ABC that they had received an "enormous amount" of death threats.

"'Everyone with Georgie's DNA should be killed' - just every kind of horrible thing you can imagine," Robert Zimmerman, the father, told ABC about the threats.