A Maryland State police trooper stands guard in Baltimore, Maryland April 28, 2015.[Photo/Agencies]

Much of the rioting occurred in a neighborhood where more than a third of families live in poverty. The violence appeared to catch city officials somewhat off-guard after a week of peaceful protests.

After Missouri was criticized for a heavy-handed response to protests over the police killing of unarmed black teenager Michael Brown in August, cities have carefully tread a line between allowing peaceful demonstrations over police brutality and preventing violence.

Answering criticism of not responding quickly enough to Monday's events, Mayor Rawlings-Blake told CNN: "This was an incident that sparked this afternoon ... I think it would have been inappropriate to bring in the National Guard when we had it under control."

Gray's family pleaded for peaceful demonstrations and after the looting started, pastors and community leaders took to the streets to try to prevent violent clashes between black youth and police.

Looters were nonchalant and showed their faces.

"We went in there and tore it up," said a 16-year-old who said he was one of the looters inside the CVS.

Just down the street from the smoldering CVS, business owner Daisy Bush, 61, said: "The sad part about it is that a lot of people from the community were up there in the CVS, stealing stuff out of it. It's a disgrace."

Earlier in the day youths threw rocks and bricks at police. Six officers were injured seriously, Baltimore Police Commissioner Anthony Batts told reporters.

"This is not protesting. This is not your first amendment rights. This is just criminal acts doing damage to a community," he said.

Baltimore has long struggled with high crime and gangs, a reputation that has made it the setting for gritty television police dramas such as "The Wire."

At Gray's funeral, speaker after speaker before the crowd packing the 2,500-seat New Shiloh Baptist Church said the world was watching to see if justice would be done for Gray.

A string of deadly confrontations between mostly white police and black men, and the violence it has prompted, will be among the challenges facing U.S. Attorney General Loretta Lynch, who was sworn in on Monday.

Lynch condemned the "senseless acts of violence" and signaled that improving relations between the police and the communities they protect will be high on her agenda.

Riots over race issues and police brutality have gripped U.S. cities in the past.

In the 1992 Los Angeles riots, more than 50 people were killed in violence set off by the acquittal of four police officers who beat black motorist Rodney King. In 1968, dozens died in riots, including several in Baltimore, after the assassination of civil rights leader Martin Luther King Jr.