Rioters trashed parts of Oakland, California, today in protest against the verdict in a controversial court case in which a white policeman shot dead an unarmed African American.

At least 100 people were arrested after looting and confrontations with police in Oakland last night and early today. The protesters ignored a plea for calm by the governor of California, Arnold Schwarzenegger.

The policeman, Johannes Mehserle, 28, was found guilty yesterday of involuntary manslaughter. The jury rejected the prosecution case that it was murder.

YouTube showed footage of Mehserle's shooting of Oscar Grant, 22, who had been lying on the platform in a railway station on 1 January 2009, surrounded by police after a fight. Mehserle claimed he had thought he had reached for an electric Taser rather than his gun.

The case became a cause celebre in the US, with its echoes of the treatment of Rodney King, a black man whose severe beating by police in LA in 1991 was captured on video. The subsequent acquittals of four LAPD officers sparked the 1992 Los Angeles riots.

Police deployed in Oakland in riot gear yesterdayand shops boarded up their windows in anticipation of a repeat of the rioting that took place in the immediate aftermath of Grant's shooting. Up to a thousand protesters took to the streets last nightand early today, some wearing masks with images of Grant's face. A banner was unfurled proclaiming "Oakland Says Guilty'.

Rioters, some dressed in black and wearing black masks, smashed shop and car windows, helped themselves to goods ranging from jewellery to groceries and trainers, and attacked police lines. Journalists were also attacked.

The Oakland police chief, Anthony Batts, told a press conference: "This city is not the wild, wild west. This city will not tolerate this activity." He blamed anarchists coming from outside Oakland. "We started taking a number of rocks and bottles. We then made a dispersal order."

Grant's family expressed disappointment with the verdict and said he should have been found guilty of murder. The verdict means the jury thought Mehserle had been criminally negligent but had not intended to kill Grant. "My son was murdered, and the law hasn't held the officer accountable the way he should be," Grant's mother, Wanda Johnson, said.

Mehserle, who is to be sentenced next month, could face anything from five to 14 years in jail. John Burris, a lawyer representing the family, described it as a "compromise verdict". Burris said: "The system is rarely fair when a police officer shoots an African American male. No true justice has been given." The trial was held in Los Angeles because of the tension in Oakland and neighbouring San Francisco.

Benjamin Todd Jealous, president of the National Association for the Advancement of Coloured People, told Reuters: "We are outraged that the jury did not find guilty of murder in a case that is so egregiously excessive and mishandled."