By Arun Kumar 

Washington, April 29 (IANS): Some 2,000 National Guardsmen and more than 1,000 police officers stood guard as a tense Baltimore City, just 60 km from the US capital, slipped under night curfew after two days of riots over the death of a black man in police custody. Presidential candidate Hillary Clinton described his death as "a tragedy that demands answers".

Hundreds of defiant protesters refused to leave the streets Tuesday night, and were facing off against gathered police officers after the riots that left more than a dozen buildings damaged, destroyed or looted and at least 20 officers injured.

At least 235 people have been arrested, 34 of them juveniles.

Protesters threw objects at the police when they first advanced on the crowd, and police responded with smoke grenades and flash-bangs about half an hour after the curfew went into effect at 10 p.m, according to various media reports.

Monday's violence in Baltimore started hours after the funeral of Freddie Gray, a 25-year-old black man who suffered a serious spine injury while in police custody. His death on April 19 at a hospital led to several days of mostly peaceful protests.

Maryland Governor Larry Hogan, who had declared a state of emergency Monday said Tuesday: "This combined force will not tolerate violence or looting, which has led to the destruction of property and put innocent Marylanders at risk."

Responding to the violence, President Barack Obama said Tuesday that "some police aren't doing the right thing" and that a lot of the tension between law enforcement and the black community stems from "a slow-rolling crisis" that has been brewing for decades.

Fixing it will require more investment in cities, criminal justice reform, better funding for education and soul-searching for some police departments, he said at a White House press conference Tuesday afternoon.

"If we really want to solve the problem, if our society really wanted to solve the problem, we could. It's just it would require everybody saying this is important, this is significant," he said.

But there was no excuse for the behaviour of "criminals and thugs who tore up" Baltimore, said Obama.

"When individuals get crowbars and start prying open doors to loot, they're not protesting. They're not making a statement. They're stealing," he said. "When they burn down a building, they're committing arson."

A number of 2016 presidential contenders also weighed in on the violence in Baltimore.

Former Secretary of State Hillary Clinton tweeted that she is praying for those in the city and Gray's family, adding that his death "is a tragedy that demands answers."

Former Maryland Governor Martin O'Malley said Monday that he was cancelling paid speeches in Europe to return to the city where he also served as mayor from 1999 to 2007.

The unrest in Baltimore follows a spate of protests across the country over the deaths of black men following encounters with police.

Time magazine recently listed 14 major instances of a white cop shooting dead a black person since 17-yearold Trayvon Martin was fatally shot on Feb 26, 2012 in Sanford, Florida.

According to another media report 154 persons have died at the hands of law enforcement officers since January with 37 killed in April alone.