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