The standoff began near the Mondawmin Mall in the northwest part of the city, the Baltimore Police Department said, as the group threw bricks, rocks and other objects at officers.
Fifteen officers were injured, six seriously, Police Commissioner Anthony Batts said in an evening news conference. Those injuries are not expected to be life-threatening.
Protesters were seen climbing on a police cruiser and damaging several others. Police tweeted that people were also looting businesses and setting cars on fire.
The clashes came hours after the funeral of 25-year-old Freddie Gray, the man who died last week after being in police custody. Police are investigation that incident.
"Several people in the group are reported to be armed with sticks, bricks, and other weapons," the police wrote on Twitter. "We are hearing reports that members of this group are now setting small items on fire."
She said a citywide curfew will be in effect beginning Tuesday night 10 p.m. to 5 a.m. The curfew will last for one week, she said, and could be extended as necessary. A 9 p.m. curfew is already in effect for children 14 and under.
Baltimore police said there was a credible threat against law enforcement officers, as members from gangs including the Bloods, Crips and Black Guerilla Family "have entered into a partnership to 'take-out' law enforcement officers," according to a news release.