Violence and looting erupted in Baltimore on Monday after the funeral of 25-year-old African-American man Freddie Gray, who died after suffering severe spinal injuries during a police arrest.

Speaking in nearby Washington, Obama condemned the rioting, but also said that a series of recent incidents – beginning last year with the police shooting of an unarmed black teenager in Ferguson, Missouri -- was worrying.

"Since Ferguson... we have seen too many instances of what appears to be police officers interacting with individuals -- primarily African-American, often poor – in ways that raise troubling questions," he said.

Obama expressed sympathy for civil rights leaders and protesters -- as well as for police on the front line of demonstrations -- and said America needed to address the strained ties between officers and blacks.

He said it was essential that "we don't just pay attention to these communities when a CVS (store) burns. And we don't just pay attention when a young man gets shot or has his spine snapped.

"I think there are police departments that have to do some soul searching.

"I think there's some communities that have to do some soul-searching. I think we as a country have to do some soul-searching. This is not new. It's been going on for decades."

In Baltimore, thousands of military and police reinforcements swarmed onto the streets on Tuesday after a night of unrest saw stores looted, more than 140 vehicles burned, 20 police wounded and 235 suspects arrested, according to police.

There was no immediate return on Tuesday to the night's chaos, but there were tense scenes when noisy supporters faced off against police lines. Arrests were made and pepper spray was deployed at least once.

Earlier, Maryland Governor Larry Hogan inspected a National Guard barricade and vowed to "make sure what happened last night in Baltimore City is not going to happen again."     

"By tonight, you're going to see an overwhelming display of people out there on the streets protecting the citizens," Hogan told reporters.

Baltimore declared an overnight curfew from 10:00 pm (local time) yesterday, to be in force for a week, and local and national leaders have appealed for calm.

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