About fifteen police officers were injured during riots on Monday.
The US state of Maryland has declared a state of emergency and deployed national guard troops on Monday evening as rioters clashed with police in Baltimore.
The violence followed the funeral of a black man whose death in custody has reignited tensions over the treatment of African-Americans by police.
Governor Larry Hogan condemned the clashes, which escalated during the afternoon as youths looted shops, torched police cars and threw rocks and bottles, injuring 15 officers.
Baltimore imposed a week-long 10pm-5am curfew from Tuesday as violence and looting continued through the night. At 2am (7am BST) police reported that several buildings and cars had just been set on fire and fire crews were being attacked.
Police arrested more than two dozen people in what are being described as the worst riots to hit the east coast port city since 1968, when racial tensions surged following the assassination of Martin Luther King. At least six officers were hospitalised after being injured.
“The national guard represents the last resort . . . to restore order,” said Mr. Hogan. “People have the right to protest and express their frustration but Baltimore city families deserve peace and safety in their communities.”
The riots, which erupted in the Mondawmin district in the west of the city, are the latest example of violent protests over the treatment of minorities — particularly African-Americans — by police. Ferguson in Missouri saw some of the worst riots in recent history last year after a white policeman shot an unarmed black man.
The Baltimore violence broke out after the funeral of Freddie Gray, 25, who died last Monday from an unexplained spine injury that occurred in custody. Six police officers have been suspended over the incident and the Justice department is investigating it.
Several buildings were ablaze on Monday evening following looting earlier in the day. Mr. Hogan said 5,000 national guard troops and another 5,000 police from other states were being called to Baltimore to tackle the riots.
People attacked a shopping mall, pharmacies, off-licences and slashed a fire hose that was being used to extinguish a blaze at a shop that had been looted and then set on fire.
It was unclear how many of the rioters were enraged about Gray’s death and how many were youths taking advantage of a chaotic situation.
Mr Hogan and Stephanie Rawlings-Blake, the Baltimore mayor, on Monday night defended their response to the violence and pushed back against suggestions that they should have acted sooner.
“It is a very delicate balancing act to make sure that you protect people’s right to free speech, their right to protest,” said Ms Rawlings-Blake, adding that it would have been inappropriate to call in the national guard over the weekend when the vast majority of the protesters were acting peacefully.
“We have seen all over the country and throughout our history what happens when you use too much force to respond to an incident,” the mayor said. “It escalates and it can be a lot worse. I didn’t want that for my city.”
All city-run primary and secondary schools will be closed on Tuesday, the authorities said.
Gray’s family had called for peaceful protests and once the looting started community leaders took to the streets in an attempt to restore calm. “I think the violence is wrong,” Gray’s twin sister, Fredericka Gray, said late on Monday, the Associated Press reported. “I don’t like it at all.”
The riots erupted as Loretta Lynch was sworn in as the first black female attorney-general in the US. On Monday evening, Ms Lynch condemned the “senseless acts of violence” that had harmed some police officers, and said those who were committing those acts did a disservice to Gray’s family.