Los Angeles, California (CNN) -- Authorities in Oakland, California, said unruly marchers were "tearing up the city" as they protested a two-year sentence for a former transit police officer convicted for killing an unarmed man.
They were throwing rocks, bottles and trash, and ripping up fences late Friday, Police Chief Anthony Batts said.
It started out as a peaceful and organized expression of grief, but by nightfall, a small number of people acted out in a violent manner, he said.
Somoeone ripped a gun from a police officer's holster, and another officer was hit by a car, according to Batts.
"I want to be clear that people have a right to assemble, a right to protest and a right to free speech," the police chief said.
"But people do not have a right to tear this city up. Oakland already has a lot of pain, and it's not fair. This city has been torn up too many times."
At least 152 people were arrested, according to Oakland police spokesman Officer Jeff Thomason.
They face charges of unlawful assembly and failure to disperse, Batts said.
The group marched down several streets and broke an agreement on what streets to rally on, Batts said.
Batts decided to declare the march an unlawful assembly after someone ripped a gun from a police officer's holster and pointed the weapon at him. The person was taken into custody, Batts said.
Johannes Mehserle, convicted of involuntary manslaughter, will get credit for time he's already spent behind bars since he was charged in the shooting of 22-year-old Oscar Grant on a train platform on January 1, 2009, a judge ruled Friday.
Mehserle could be released from custody in about seven months, according to sentencing guidelines provided by the prosecution.
Protesters Friday broke windows and jumped on vehicles, Batts said. He estimated the crowd at between 300 and 500 people.
Mayor Ron Dellums appealed to citizens to protest in a nonviolent manner.
Grant's mother, Wanda Johnson, appeared stunned as she left the courtroom. Her family's lawyer said she was appalled. Johnson had asked the judge to sentence him to the maximum 14 years in prison. She and four other family members who spoke at the sentencing hearing called Mehserle "a murderer."
"This is a slap in the face, a punch in the stomach," said John Burris, the Grant family attorney.
Prosecutors had asked for prison time, while the defense had argued for probation.
After the July verdict, police in downtown Oakland arrested dozens of angry protesters on a variety of charges, including failure to disperse, resisting arrest, burglary, vandalism and assaulting a police officer. The city planned to have extra officers on hand Friday in case they were needed, said police spokeswoman Holly J. Joshi.
Mehserle told Los Angeles County Superior Court Judge Robert Perry before sentencing Friday that he would be willing to go to prison if the sentence made his city and family safer.
"I shot a man," he said. "I killed a man. It should not have happened."
A conviction for involuntary manslaughter normally carries a four-year sentence, but the judge had the option of adding an "enhancement" that could have made the sentence 14 years because a firearm was used in commission of a crime.
Mehserle, who was on duty as a Bay Area Rapid Transit police officer when the shooting occurred, said at the trial that he intended to draw and fire his Taser rather than his gun. The jury acquitted him of the more serious charges of second-degree murder and voluntary manslaughter.
Grant family members expressed outrage after the verdict in July.
"My son was murdered. He was murdered. He was murdered. My son was murdered," Johnson said.
The shooting was captured on a bystander's cell-phone video camera, and the video was widely circulated on the Internet and on news broadcasts. It spurred several protests and riots in and around Oakland.
Mehserle is white and Grant was African-American.
Grant's mother said her son and other African-Americans have too long been the victims of police abuse and a biased judicial system.
The trial had been moved from California's Alameda County to Los Angeles because of pretrial publicity.
The former officer apologized to the public and described his memories of the moments after the shooting in a handwritten letter obtained by CNN after the verdict.
"For now, and forever, I will live, breathe, sleep, and not sleep with the memory of Mr. Grant screaming 'you shot me' and me putting my hands on the bullet wound thinking the pressure would help while I kept telling him 'you'll be okay,'" Mehserle wrote in the letter. "I tried to tell myself that maybe this shot would not be so serious, but I recall how sick I felt when Mr. Grant stopped talking, closed his eyes and seemed to change his breathing."
CNN's Stan Wilson and Phil Gast contributed to this report.