Thousands of people marched peacefully through downtown Baltimore on Saturday to protest the unexplained death of a black man in police custody but pockets of violence erupted when a small group smashed windows and threw bottles at officers.
At least 2,000 demonstrators attended the march to City Hall, the largest turnout since 25-year-old Freddie Gray died a week ago.
As darkness fell, about 100 protesters splintered from the group and threw bottles, metal barricades and other objects at police officers and their cruisers, authorities said.
The windows of several businesses were smashed, Police Commissioner Anthony Batts said.
Police arrested 12 people who ignored orders to disperse, Batts said. At least one officer was hurt in the skirmishes.
Fredericka Gray, Freddie's twin sister, joined Baltimore Mayor Stephanie Rawlings-Blake at a news conference where she urged people to keep calm.
"Freddie's father and mother do not want violence, violence does not get justice," she said.
Gray died April 19 after suffering a spinal injury while in police custody.
With his death, Gray joined a long list of black men who have died under questionable circumstances during police encounters in recent months. The highly publicized incidents have triggered an outcry over the use of force by law enforcement against African-Americans.
Last year, weeks of protests followed the fatal shooting of unarmed black teen Michael Brown by a white officer in Ferguson, Missouri, and the chokehold death of Eric Garner in New York City.
Six Baltimore police officers have been suspended in the Gray case, and an internal police investigation is under way.
Baltimore Mayor Rawlings-Blake, who has called for answers in Gray's death, said agitators at the Saturday demonstration disrupted the otherwise peaceful political action.
"I am profoundly disappointed to see the violence in our city this evening," she said.
Much of the violence occurred near the Camden Yards ballpark, where the Baltimore Orioles played the Boston Red Sox as scheduled. Towards the end of the game, fans were told to stay in place because of safety concerns.
Protesters are calling for the prosecution of the six officers involved in Gray's arrest and a reform of policing tactics.
On Friday, Commissioner Batts said the officers repeatedly failed to give Gray medical assistance and disregarded other regulations.
Gene Ryan, president of the Baltimore police union, said in a statement the commissioner's comments were premature and "appear to be politically driven."