LOS ANGELES/OAKLAND, California (Reuters) - A white former transit police officer was found guilty of involuntary manslaughter in a videotaped shooting death of an unarmed black man last year in Oakland, California, sparking a wave of looting and destruction in the city on Thursday.

A demonstrator is arrested during a protest against the verdict in the case of Bay Area Rapid Transit (BART) officer Johannes Mehserle in Oakland, California July 8, 2010. Mehserle, a white police officer, was found guilty of involuntary manslaughter on Thursday in the videotaped shooting death of Oscar Grant, an unarmed black man, that triggered riots in Oakland, California last year. REUTERS/Robert Galbraith

The verdict prompted a peaceful protest by up to 1,000 people in downtown Oakland, which gave way after nightfall to some people looting stores, smashing car windows, throwing powerful fireworks at police and lighting fires in trash cans.

The police, numbering in the hundreds, made more than 50 arrests, but Oakland police expected that figure could double.

“This city is not the wild, wild West,” Oakland Police Chief Anthony Batts told a televised news conference. “This city will not tolerate this activity.”

A Los Angeles jury deliberated for about six hours over two days before reaching their decision about the shooting on a train platform in Oakland, indicating they deemed it a tragic accident rather than the intentional act of a rogue cop.

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

But prosecutors, who sought a conviction for second-degree murder, said Mehserle had “lost all control” and shot Grant on purpose because he thought Grant was resisting arrest.

Jurors can render an involuntary manslaughter conviction if they believe the defendant lacked an intent to kill but engaged in conduct so grossly negligent that it amounts to a crime.

It generally carries a sentence of two to four years in prison, but the jury also accepted a sentencing “enhancement” for Mehserle’s use of a handgun.

“We are outraged that the jury did not find guilty of murder in a case that is so egregiously excessive and mishandled,” said Benjamin Todd Jealous, head of the National Association for the Advancement of Colored People (NAACP).

Mehserle, who had been free on $3 million bond, showed no reaction as the verdict was read and was immediately taken into custody. The former police officer for the Bay Area Rapid Transit (BART) rail system faces sentencing on August 6.

FAMILY FURY

Relatives of Grant, a young father who worked as a grocery store butcher, reacted with outrage.

“My son was murdered, and the law hasn’t held the officer accountable the way he should be,” Grant’s mother, Wanda Johnson, shouted outside the courthouse.

About 1,000 people gathered in a peaceful protest in downtown Oakland on early Thursday evening. Many expressed anger, with a huge banner strung over an intersection on a traffic light pole reading “Oakland says Guilty.”

“It’s unbelievable this guy is getting less jail time than someone who wrote a bad check,” said Barbara Plantiko, a 41-year-old immigration lawyer at the protest. “I just don’t buy he got confused. I don’t think that it was an accident.”

Some protesters wore masks depicting Grant’s face.

Slideshow ( 4 images )

Looters targeted stores selling jewelry and beauty supplies and grabbed shoes from a Foot Locker store in downtown Oakland, while the phrase “Riot for Oscar” was spray-painted on a bank building, according to a Reuters eyewitness.

“It’s definitely chaotic,” said Sgt. J.D. Nelson, an Alameda County Sheriff spokesman, adding that Oakland’s police were being helped by officers from other municipalities.

The killing had unleashed charges of police brutality and unrest in Oakland in January 2009, when people smashed windows and set cars on fire, leading to about 100 arrests then.

Video footage of Grant lying face down as Mehserle shot him in the back was taken by onlookers and shown over the Web and television. Mehserle was seen holstering his gun immediately afterward and putting his hands on his head as in disbelief.

The judge in the case, which was moved to Los Angeles due to heavy pretrial publicity in the Bay Area, held there was too little evidence to show the killing was premeditated, ruling out a first-degree murder conviction.

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