Oakland, California (CNN) -- Hundreds stormed the streets of downtown Oakland on Thursday night after a verdict in the trial of a white former police officer who fatally shot an unarmed black man.
Johannes Mehserle was found guilty of involuntary manslaughter Thursday, a conviction that usually carries a maximum four-year sentence.
But some in Oakland expected a tougher penalty for the former police officer, and took to the streets in protest.
Crowds broke the glass of a Foot Locker and other stores. Others threw sneakers out of the store as police wearing gas masks stormed the area.
Oakland Police Chief Anthony Batts said there were 50 arrests and he expected the number to double by the end of the night.
At the high point of the protests about 8 p.m. (11 p.m. ET), there were an estimated 800 people in the streets, Batts said.
By 10:30 p.m, there were about 75 left, Batts said.
The protests were contained to the downtown area, police said.
Oakland Mayor Ron Dellums downplayed the demonstrations.
"People were preparing for everything to explode, but I am extremely happy that so far it has not, and I hope that it doesn't," Dellums said. "We're not going to tear our own community apart, because we've got issues that we've got to deal with".
Oakland police had prepared for protests and ways of quelling the demonstrations days before the verdict.
Mehserle could have been found not guilty, guilty of second-degree murder, guilty of voluntary manslaughter -- or guilty of involuntary manslaughter -- as the jury decided.
Mehserle, a Bay Area Rapid Transit police officer at the time of the incident, was accused of shooting 22-year-old Oscar Grant on an Oakland train platform on January 1, 2009.
The trial was moved from Alameda County to Los Angeles because of pre-trial publicity.
Mehserle, wearing a gray suit, blue shirt and red tie, showed no emotion during the reading of the verdict. The former officer did not saying anything to Superior Court Judge Robert Perry or attorneys. About a dozen Los Angeles County deputies escorted the handcuffed defendant out of the courtroom after the verdict was announced.
Outside the courtroom, Grant family members expressed outrage at the verdict.
"My son was murdered. He was murdered. He was murdered. My son was murdered," said Grant's mother, Wanda Johnson. "The system has let us down, but God will never ever let us down".
Johnson and other speakers said African-Americans have been the victims of police abuse and a biased judicial system. She said Mehserle wasn't found accountable.
"We couldn't get even six hours of deliberations," said Johnson, who accused jurors of being unfair.
Mehserle, who was on duty when the shooting occurred, said at the trial that he intended to draw and fire his Taser rather than his gun, CNN affiliate KTVU reported.
Sentencing is set for August 6. Involuntary manslaughter carries a maximum sentence of up to four years in prison under California law. But the judge could add an "enhancement" that could provide a longer sentence because a firearm was used.
Members of the jury, which included no African-Americans, said they were unanimous in their decision. Their finding indicates that Mehserle was criminally negligent.
The shooting was captured on a bystander's cell-phone video camera. The video was widely circulated on the Internet and on news broadcasts, and it spurred several protests in and around Oakland.
Bay Area Rapid Transit police were called to Oakland's Fruitvale station on January 1, 2009, after passengers complained about fights on a train. Officers pulled several men, including Grant, off the train when it arrived at Fruitvale.
The video showed Mehserle pulling his gun and fatally shooting Grant in the back as another officer knelt on the unarmed man.
Mehserle resigned his position a few days after the incident and was later arrested in Nevada.
