LOS ANGELES (Reuters) - A white transit police officer was found guilty of involuntary manslaughter on Thursday in the videotaped shooting death of an unarmed black man that triggered a night of rioting in Oakland, California.

The defendant in the racially charged trial, Johannes Mehserle, 28, testified that he mistakenly drew his pistol instead of his electric Taser weapon and shot Oscar Grant, 22, while trying to subdue him during a New Year’s Day 2009 confrontation.

But prosecutors said in closing arguments that Mehserle “lost all control” and intentionally shot Grant because he was resisting arrest.

The Los Angeles County jury of four men and eight women deliberated for about six hours over two days before reaching their verdict, indicating they essentially believed Mehserle’s account that he shot Grant accidentally.

Juries can find a defendant guilty of involuntary manslaughter if they believe he lacked the intent to kill but that his actions were so grossly negligent that he should be held criminally responsible for them.

Legal experts have said involuntary manslaughter is generally punishable by two to four years in prison. It is rare for a law enforcement officer to be charged with murder in connection with an on-duty shooting.

Police in Oakland, across the Bay to the east of San Francisco, moved to a tactical alert status as they braced for the possibility of renewed violence following the verdict. But civic leaders appealed for calm.

Demonstrations by supporters of Grant, a young father who worked as a grocery store butcher, were planned in Oakland and Los Angeles.

“We don’t know if we’re going to have a riot or a celebration, but either way we’re going to have one,” protester Cindy Delgado said outside the downtown Los Angeles courthouse before the verdict was announced.

“I’m concerned about riots. I don’t want to be hit by a bottle,” said Francisco Raygoza, 30, an accountant leaving work in San Francisco. “Our office manager said leave as soon as you can.”

Video footage of the slaying shown widely over the Internet and television, appeared to show Grant lying face down on the train platform when he was shot in the back. Mehserle was seen holstering his gun immediately afterward and putting his hands on his head as in disbelief.

The killing unleashed charges of police brutality and a night of civil unrest in Oakland, where demonstrators smashed store windows and set cars on fire. Police arrested over 100 people on charges of vandalism, unlawful assembly and assault.

The Alameda County Superior Court judge in the case, which was moved to Los Angeles because of heavy pretrial publicity in Oakland, ruled that the jury could not consider a first-degree murder charge. Judge Robert Perry held there was too little evidence to show the killing was premeditated.

Had he been convicted of second-degree murder, Mehserle faced a sentence of 15 years to life in prison. The jury could alternatively have found him guilty of voluntary manslaughter or acquitted him entirely.