Protesters smashed windows and hurled objects at officers as the biggest protest yet over the death of a young black man in police custody turned violent.
At least 2,000 people had joined a peaceful 90-minute rally at Baltimore city hall demanding justice for Freddie Gray, 25, who died last Sunday from spinal injuries, a week after his arrest for carrying a knife.
But after the rally the mood shifted dramatically when some groups of protesters fanned out across the city and disturbances were reported.
Baltimore police said on Twitter that some protesters threw objects at officers and broke windows.
Local television showed footage of a protester throwing a crowd-control barrier toward officers.
Protesters also jumped on some police cars, breaking their windshields.
At various times, protesters faced off against officers in front of Camden Yards, home of the Orioles baseball team whose game against the Boston Red Sox began as scheduled.
At least 12 arrests were made during the protest, police said.
Speakers at the rally called for President Barack Obama to launch a national inquiry into police misconduct, following a series of fatal confrontations between white police officers and black men and boys.
“It has to stop. It really has to stop because it could have been any one of us,” a young male adult member of Mr Gray’s extended family told the crowd.
Last year, weeks of protests followed the shooting death of unarmed black teen Michael Brown in Ferguson, Missouri, and the death of Eric Garner, a black man in New York City who was placed in a chokehold during an attempted arrest.
On Friday, Baltimore’s police commissioner conceded that police had failed to provide Mr Gray with timely medical attention for a spinal injury he had suffered sometime after he was apprehended and put inside a transport van.
Police have not explained how he sustained the injury.
Six Baltimore police officers have been suspended in the case, and an internal police investigation is under way.
The department will turn over the findings to state prosecutors and an independent review will follow.