The family of a black teenager who was shot dead in Missouri demanded justice after a grand jury decided not to indict the policeman who killed him, sparking nationwide protests.

Attorneys for Michael Brown's family criticised the process that led to police officer Darren Wilson not standing trial as unfair and broken, saying that a special prosecutor should have been appointed to deal with the case.

"We object to this process because all across America ... young people of colour are being killed by police officers, and the local prosecutors put together this very 'unbiased' grand jury, and it continues to yield the same results," Benjamin Crump, a lawyer said, The Times reported.

"We are not insane. We know that our children deserve equal justice, just as any American," Crump said, flanked by Brown's father.

Brown, 18, was unarmed when he was shot dead in Ferguson on 9 August during a confrontation with police.

The incident stoked widespread protests which gained new momentum earlier this week as St. Louis County prosecutor Robert McCulloch announced the grand jury found there was not enough evidence to proceed against Wilson earlier this week.

Following the ruling, dozens buildings and vehicles were set on fire by rioters in Ferguson and the protest have since spread throughout the country with incidents reported in Oakland, California.

Meanwhile Missouri Governor Jay Nixon said more than 2,200 national guard troops are being deployed to quell unrest in St Louis.

In an exclusive interview ABC News, Wilson said he has a clear conscience "because I know I did my job right."