Oakland, California (CNN) -- Police arrested dozens of people in downtown Oakland, California, Thursday night after hundreds protested the verdict in the trial of a white former police officer who fatally shot an unarmed black man, Oakland police officials told CNN.

Police issued a variety of charges, including failure to disperse, resisting arrest, burglary, vandalism and assaulting a police officer. Police initially reported 83 people had been arrested, but later reduced the number to 78.

The former officer, Johannes Mehserle, was found guilty of involuntary manslaughter Thursday, a conviction that usually carries a maximum four-year sentence.

But some in Oakland hoped for conviction on a charge that would carry 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.

At the high point of the protests, about 8 p.m., there were an estimated 800 people in the streets, Oakland Police Chief Anthony Batts said.

By 10:30 p.m., there were about 75 left, he said.

The protests were contained to the downtown area, police said.

Police said they were still collecting information on the total number of businesses that were vandalized.

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, as the jury decided, guilty of involuntary manslaughter.

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 pretrial 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 say 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.

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.

The video showed Mehserle pulling his gun and fatally shooting Grant in the back as another officer knelt on the unarmed man.

Mehserle and other Bay Area Rapid Transit police had been called to Oakland's Fruitvale station after passengers complained about fights on a train. Officers pulled several men, including Grant, off the train when it arrived at Fruitvale.

Mehserle said at the trial that he intended to draw and fire his Taser rather than his gun.

He resigned his position a few days after the incident and was later arrested in Nevada.

CNN's Sara Weisfelt contributed to this report.