Protests have turned violent after a grand jury decided not to indict a white police officer in the fatal shooting of an unarmed black teenager in Ferguson, Mo., in a case that has elevated tensions between the community and local law enforcement.

That means officer Darren Wilson, 28, will not be charged in the death of Michael Brown, based on the decision of the 12-person panel convened by St. Louis County.

Police in tactical gear used pepper spray and smoke bombs against protesters as demonstrations grew violent Monday night.

One police cruiser was reportedly vandalized with rocks and, after being abandoned, was apparently set on fire.

A second cruiser was later set on fire, along with a Little Caesar’s pizza shop, which looked to have been completely burned down.

Several gunshots were also reportedly heard.

Some protestors chanted “Hands up, don’t shoot” as they marched peacefully through the streets, while others began looting local shops.

Several people were seen climbing out through the windows of the store from which Brown was reported to have stolen a box of cigars before the fatal shooting occurred.

Leslie McSpadden, Brown’s mother, was screaming when she learned the news from a nearby hotel room, according to the family’s lawyer.

Ben Crump, the lawyer, said that the family had deep mistrust of the prosecutor’s office from the beginning.

Crump also released a written statement from Brown’s parents shortly after the decision was announced around 8:30 p.m. local time.

"We are profoundly disappointed that the killer of our child will not face the consequence of his actions,” said the statement. “While we understand that many others share our pain, we ask that you channel your frustration in ways that will make a positive change.”

“Join with us in our campaign to ensure that every police officer working the streets in this country wears a body camera,” they said.

The grand jury -- which considered photographs, medical reports, eye-witness accounts and four hours of voluntary testimony from Wilson -- was made up of nine white people and three black people.

Wilson’s defence team issued a statement saying the officers wanted to “thank those who have stood by his side throughout the process.”

“Law enforcement personnel must frequently make split-second and difficult decisions. Officer Wilson followed his training and followed the law,” his lawyers said.

“We recognize that many people will want to second-guess the grand jury’s decision. We would encourage anyone who wants to express an opinion do so in a respectful and peaceful manner.”

A decision in favour of the officer was expected to bring protests. Earlier in the evening, Missouri Gov. Jay Nixon outlined the planned response.

“The police have been trained to make sure we are respecting people’s rights,” he said, “however, on the other side, if people are violent or threaten property, resources will be used to manage the issues.”

“Our focus today, in the short run here is to protect lives, protect property and to protect speech,” he said, “and in the longer run, to find paths for progress.”

And after the decision was announced, President Barack Obama appealed for peace.

“We are a nation built on the rule of law, so we need to accept that this decision was the grand jury’s to make,” he said.

He said it was “understandable reaction” that some Americans would be disappointed or angry but emphasized Brown’s father’s wishes for “positive change.”

“Michael Brown’s parents have lost more than anyone,” he said. “We should be honouring their wishes.”

What exactly happened when Brown, 18, was killed by Wilson on Aug. 9 in the small city of Ferguson has been widely disputed. Many believe race was a factor.

Two men said they witnessed Brown raise his hands when the officer approached with his weapon and repeatedly fired.

Some interpret Brown’s gesture as surrendering. Officer Wilson, however, said that he feared for his life during a struggle over his gun that occurred when he was pinned up against his police car.

Brown’s death ignited simmering tensions between the St. Louis suburb’s predominantly black population and the municipality’s mostly white police force, who many believe used excessive force.

Brown’s death was followed by days of protests and looting that began the next day. Some protesters stood atop police cars. Others made off with bags of food, toilet paper and alcohol.

On Aug. 11, the FBI opened an investigation into Brown’s death, but it was not enough to quell the protests.

Three days later, the Missouri Highway Patrol took over security in Ferguson and officers were equipped with armoured vehicles, body armour and assault rifles.

On August 16, Missouri Gov. Jay Nixon declared a state of emergency.

Two days later the National Guard was called to Ferguson to restore order.

Protests have continued, often with marchers raising their arms in the air and saying “don’t shoot.”

The case generated such interest that both the FBI and U.S. Justice Department are investigating whether civil rights have been breached.

Last week, Brown’s father issued a statement thanking protesters for fighting to end racial profiling and police intimidation, but urged them to remain peaceful regardless of the grand jury's decision.

"I do not want my son's death to be in vain," he said.

On Monday, Brown’s family reiterated their request that protests remain peaceful. They also asked that supporters observe four-and-a-half minutes of silence after the announcement, as a reminder of the four-and-a-half hours that Brown’s body remained in the street after he was shot.

An autopsy conducted by a military doctor on behalf of the Justice Department found Brown was shot six times.

The Ferguson Police Department has four black members among its 53 officers.

In 2010, the City of Ferguson had about 21,000 residents, more than two-thirds of them black, according to the U.S. Census.

The St. Louis County prosecutor’s office has said the grand jury’s proceedings have been recorded so that they can be released to the public.