A protestor on a bicycle thrusts his fist in the air next to a line of police, in front of a burning CVS drug store, during clashes in Baltimore, Maryland April 27, 2015. [Photo/Agencies]

"They don't want this movement nationally to be marred by violence," he said. "It makes no sense."

Police urged parents to locate their children and bring them home. Many of those on the streets appeared to be African-American youths, wearing backpacks and khaki pants that are a part of many public school uniforms.

Later in the day, people began looting clothing and other items from stores at the mall, which became unprotected as police moved away from the area. About three dozen officers returned, trying to arrest looters but driving many away by firing pellet guns and rubber bullets.

While the violence had not yet reached City Hall and the Camden Yards area, the Orioles baseball team canceled Monday's game for safety precautions.

Many who had never met Gray gathered earlier in the day in a Baltimore church to bid him farewell and press for more accountability among law enforcement.

The 2,500-capacity New Shiloh Baptist church was filled with mourners. But even the funeral could not ease mounting tensions.

Police said in a news release sent while the funeral was underway that the department had received a "credible threat" that three notoriously violent gangs are now working together to "take out" law enforcement officers.

A small group of mourners started lining up about two hours ahead of Monday's funeral. Placed atop Gray's body was a white pillow with a screened picture of him. A projector aimed at two screens on the walls showed the words "Black Lives Matter & All Lives Matter."