Around 2,000 National Guard troops were dispatched to the St Louis area helped police stave off a second night of rioting and arson as sympathy protests spread to several US cities after a grand jury declined to indict a white policeman in the fatal shooting of an unarmed black teenager.

President Barack Obama appealed for dialogue, and his attorney general promised that a federal probe into the shooting of 18-year-old Michael Brown in August would be rigorous.

Officer Darren Wilson, the policeman who shot him, said his conscience was clear.

Despite a beefed-up military presence in Ferguson, a police car was torched near City Hall as darkness fell, and police fired smoke bombs and tear gas to scatter protesters. A crowd of demonstrators later converged near police headquarters, scuffled with officers who doused them with pepper spray, then smashed storefront windows as they fled under orders to disperse.

"We saw some protesters out there that were really out there for the right reason. Unfortunately, there seems to be a few people who are bent on preventing this from happening in the most ideal way that it could," St Louis County Police Chief Jon Belmar told reporters early Wednesday

An enlarged contingent of National Guard troops surrounded businesses damaged in Monday's violence. Groups of men also gathered on the roofs of some boarded-up stores to protect the buildings from further damage. Armed with fire extinguishers and, one said, guns, they planned to stay all night.

Elsewhere, protests swelled from Los Angeles to Washington, DC, on Tuesday.

In New York, police used pepper spray to control the crowd after protesters tried to block the Lincoln Tunnel and Triborough Bridge and marched to Times Square. Several hundred also marched in Harlem, chanting "Racist police!"

Four people were arrested for blocking a roadway in Denver, where police said several hundred people turned out for a protest march. In one of the night's biggest rallies, an estimated 1,500 people took to the streets of Boston, though police there reported just a handful of arrests. Inmates at a correctional facility in Boston taped Brown's name on a window in solidarity with protesters who marched outside.

Missouri Governor Jay Nixon said about 2,200 National Guard troops were to be deployed to the Ferguson area by late Tuesday, more than triple the number from the day before, to help protect homes and businesses and to support local law enforcement.

The grand jury decision shifted the legal spotlight to a US Justice Department investigation into whether Wilson violated Brown's civil rights by intentionally using excessive force and whether Ferguson police systematically violate rights through excessive force or discrimination.

Wilson, who could have faced charges ranging from involuntary manslaughter to first-degree murder, told ABC News there was nothing he could have done differently in his confrontation with Brown that would have prevented the teenager's death. "The reason I have a clean conscience is because I know I did my job right," he said, adding he would have acted no differently had Brown been white.

Wilson's lawyer, Jim Towey, later told CNN that his client's life as a police officer was over.

Documents released by prosecutors said that Wilson, who was placed on administrative leave after the shooting, told the grand jury Brown had tried to grab his gun, and that the officer felt his life was in danger when he fired. - Reuters