Updated 7.40pm: The streets of Ferguson, Missouri, are decidedly calmer after two days of nationwide protests against the decision not to prosecute a white policeman who shot dead an unarmed black teenager.

Just a few dozen protesters and clergy braved rain and light snow to protest outside the police department in the St Louis suburb, where 18-year-old Michael Brown was killed on August 9.

A Missouri grand jury decided on Monday not to prosecute Officer Darren Wilson, who fired the fatal shots - a move that inspired coast-to-coast anger in the US.

Soldiers deployed in Ferguson 

More than 2000 soldiers have been deployed in the US town of Ferguson on a second night of protest after a grand jury chose not to charge a white policeman who shot dead an unarmed black teen.

After a night of racially-charged unrest following Monday's decision, Missouri governor Jay Nixon said the National Guard force in the Ferguson area would be tripled to halt the looting and arson.

At the Ferguson police station on Tuesday night, a cordon of 50 riot police faced off across a road with around 100 noisy protesters chanting and waving placards, including one that read: "We will not be silenced."

Inside the perimeter of the station, National Guardsmen equipped with riot shields and batons could be seen preparing for the night.

The protest crowd was smaller than it had been on Monday, and there was no initial sign of violence, but some masked agitators could be seen on the fringes of the demonstration.

"Lives and property must be protected. This community deserves to have peace," the governor said, as anger mounted nationwide and fresh protests took place in major cities.

Marchers disrupted traffic on bridges and the Lincoln Tunnel in New York city - leading to a number of arrests - and crowds took to the streets in downtown Washington, near the White House. Protests were also held in Boston, Philadelphia, Baltimore, Portland, Seattle, Los Angeles and Chicago, among others.

US President Barack Obama decried the violence and called for rioters to be prosecuted, while acknowledging the deep-rooted frustrations of minorities who feel they are unfairly treated by police.

"There are productive ways of responding and expressing those frustrations and there are destructive ways of responding," he said.

"Burning buildings, torching cars, destroying property, putting people at risk. That's destructive and there's no excuse for it. Those are criminal acts."

Lawyers for the family of slain youth Michael Brown denounced the prosecutor whose grand jury hearing found that police officer Darren Wilson had killed the 18-year-old in self-defence.
"This process is broken. This process should be indicted," Brown family lawyer Benjamin Crump told a news conference.

Crump criticised the way Wilson had not been cross-examined when he appeared before the grand jury, which decided not to indict him over the August 9 shooting.

But in his first televised comments since the incident, Wilson told ABC News he had feared for his life during the confrontation, believing Brown was attempting to wrestle his gun away from him.

"I can feel his hand trying to come over my hand and get inside the trigger guard and try to shoot me with my own gun," Wilson said.

Asked if he believed he would have acted the same way if Brown was white, Wilson responded: "No question."

The officer said he was comfortable that he had acted correctly.

"I don't think it's haunting. It's always going to be something that happened," he said, adding that his conscience was clear because "I know I did my job right".

Civil rights firebrand Al Sharpton said the Brown case renewed a nationwide fight for greater police accountability.

"This is not a Ferguson problem.... This is a problem all over the country," Sharpton said. "We may have lost one round but the fight is not over. They have broken our hearts, but not our backs."

Separate protests flared, meanwhile, in Cleveland, Ohio, following the fatal shooting by police of a 12-year-old black boy holding a replica gun at the weekend.

Thousands have protested in London in sympathy with demonstrations across the US over the killing of a black teenager by a white police officer.

The decision not to prosecute the officer has sparked anger and unrest in cities across the US.