Violence erupted between groups of young people and police in Baltimore on Monday following the funeral of a black man, Freddie Gray, who died in police custody.
At least seven police officers have been injured during the clashes, some have broken bones.
Protesting youths threw bricks and stones, burned patrol cars, stoned a city bus and passing police vehicles outside the Mondawmin Mall in northwest Baltimore just a few blocks from the site of the funeral. They also stormed a pharmacy in the area.
Protesters ignored the calls to disperse by a police helicopter.
Baltimore Police Captain Eric Kowalczyk said at a police briefing that one of the injured police officers was unresponsive and had several broken bones.
Earlier, Baltimore police reported receiving a "credible threat" that several gangs were planning to try to "take out" law enforcement officers.
However, police department did not give any details on the source of the threat or its relation to Gray’s death, but urged the officers to take necessary precautions to ensure their safety.
Today’s funeral was preceded by days of protests over Gray's death in the latest outcry over U.S. law enforcement's treatment of minorities.
Thousands of people including many members of congress attended the ceremony in New Shiloh Baptist Church for Freddie Gray, who died on April 19, one week after his spine injured during a police arrest.
"At 25, he had to feel that the walls were closing in," said the Rev Jamal Bryant of Empowerment Temple AME Church, who delivered the eulogy. "He had to feel boxed in" by poverty and lack of opportunity, just like many other young black men.
"When that black boy got out of the casket, that black boy said, 'No justice!'" Bryant said. The crowd roared in response, "No peace!"
"No justice, no peace" has been a rallying cry in Baltimore protests against police brutality.
U.S. Representative Elijah Cummings of Baltimore, one of numerous politicians at the funeral, vowed to see justice to be done.
"It is on our watch," Cummings said. "We will not fail you."
Also at the funeral were Mayor Stephanie Rawlings-Blake and civil rights leader the Rev Jesse Jackson Sr, Gray's five sisters and other family members, and clergy from across the predominantly black city of 620,000 people.
President Barack Obama sent Broderick Johnson, the head of his initiative for minority males, to attend the funeral.