Lawyers for Michael Brown's family say the process that led to a white officer not being indicted in the fatal shooting of the unarmed, black 18-year-old is "unfair and broken".

Benjamin Crump said on Tuesday that the family's legal team objected to St Louis County Prosecutor Bob McCullough's decision to call a grand jury in the case and not appoint a special prosecutor.

Speaking at a news conference in Ferguson, where Brown was shot on August 9, Crump also called for protests to remain peaceful.

The riots overnight on Monday had been widely condemned and led to scores of people being arrested.

"No-one is going to condone violence," he said. "Michael Brown Senior has eloquently called for peace and calm.

"We all have a responsibility to protect the community."

Missouri governor, Jay Nixon, said more than 2,200 National Guardsmen will be in place in the region near Ferguson on Tuesday night in the event of more violence. Nixon said 700 guardsmen were in the area on Monday night, when more than a dozen buildings were set on fire and others vandalised.

Several protests broke out for a second day in the St Louis area and other cities. About 300 people marched from a park to the St Louis courthouse, chanting "You didn't indict. We shall fight". Police used pepper spray and arrested several demonstrators who blocked major intersections in St Louis.

In his first public statements during an interview with ABC News, white police officer Darren Wilson said he has a clean conscience because "I know I did my job right".

Obama condemns violence

President Barack Obama condemned the violence, saying they are criminal and those responsible should be prosecuted.

But America's first black president said he understands that many people are upset by the grand jury decision. He said that their frustration is rooted in a sense that laws are not always being enforced "uniformly and fairly" in communities of colour.

"Burning buildings, torching cars, destroying property, putting people at risk ... there's no excuse for it," Obama said.

Obama urged parties aggrieved by events in Ferguson to work peacefully to achieve change, saying the case had exposed "an American problem".

Civil rights activist Al Sharpton, who also spoke at the news conference, said McCullough had gone out of his way to discredit Michael Brown.

"I have never seen a prosecutor hold a press conference where he went out of his way to discredit the witness," the reverend said.

"Have you ever heard a prosecutor explain to the press why the one who did the killing is not going to trial, but the victim is guilty of several things that no-one has established?"

'Michael Brown's law'

Sharpton said the prosecutor had "methodically tried to discredit witnesses still needed in the ongoing Federal investigation". The Federal investigation into the shooting is still active.

Lawyer Crump called for the introduction of 'Michael Brown's law' which would mean all police within the US would be required to wear a personal video camera so their actions were "transparent".

"We could see what the outcome was going to be, and that is what occurred last night," he said.

"We object to this process because, all across America, young people of colour are being killed by police officers and local prosecutors put together these unbiased grand juries and it continues to yield the same result."

Lawyer Anthony Gray said the decision was a "direct reflection of the presentation of the evidence," and criticised what he called "cynicism" in the questions found in the grand jury documents, which were released on Monday night.

McCullough's impartiality has previously been questioned due to the fact that his police officer father was shot and killed by an African American in 1964.