Thousands of people rallied in US cities including Los Angeles and New York to protest against a grand jury's decision not to indict a white police officer who killed an unarmed black 18-year-old in Ferguson, Missouri.

Gunshots rang out and police lobbed tear gas at an angry crowd in suburban St Louis after the grand jury decision was announced late on Monday.

Angry protesters overran barricades and taunted police in Ferguson, with some chanting "murderer" and others throwing stones and bottles, as police car windows were smashed and protesters tried to set vehicles ablaze.

Officers responded by firing what authorities said was smoke and pepper spray into the crowd, as some in the crowd tried to stop others from taking part in the violence.

The protesters ignored calls by US President Barack Obama on Monday night to accept the grand jury's decision.

"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," Obama said. He echoed Brown's parents in calling for any protests to be peaceful.

Al Jazeera's Daniel Lak reporting from Ferguson "I've covered riots and unrest in many countries and this was far from the worst I've seen. Also, I was impressed, at least in the few areas I was able to observe, by the restraint shown by police. At least in comparison to what went on here in August. There was no repeat of the military-style tactics, police dressed as soldiers and carrying infantry rifles. I saw some armoured vehicles but where I was at least, they didn't drive at protestors in a threatening manner. They hung back behind police lines. Of course this is just the first hours of what could be an ongoing situation. But talking to people who live here, angry as they are at the grand jury decision, most seem to want peaceful protests on both sides. As one woman said to us during the evening "looting and burning don't help us tell the world how bad things are here. They just make it look like our fault. It's not our fault. We shouldn't have to live this way. Michael Brown should still be alive."

Activists had been planning to protest even before the nighttime announcement that officer Darren Wilson would not be charged in the shooting death of Michael Brown on August 9.

Wilson shot Brown 12 times according to evidence presented to the grand jury, with witness reports differing as to whether and when Brown had his hands raised.

'Black lives matter'

In New York City marchers chanting "Black lives matter" snarled traffic on Broadway through Times Square.

In Chicago, demonstrators walked up Lake Shore Drive carrying banners that read "Justice for Mike Brown".

Police in Ferguson used smoke canisters and trucks to force waves of violent protesters down the street away from the police building soon after sporadic gunshots were heard. Flames from a burning car rose into the night sky.

Whistles pierced the air as some of the hundreds of protesters tried to keep the peace, shouting, "Don't run, don't run."

Police who formed a wall of clear riot shields outside the precinct were pelted with bottles and cans as the crowd surged up and down the street immediately after authorities said the grand jury had voted not to indict Wilson.

"Murderers, you're nothing but murderers," protesters in the crowd shouted. One woman, speaking through a megaphone said, "Stinking murderers."

Dozens of police and military vehicles were poised for possible mass arrests not far from the stretch of Ferguson streets that saw the worst of the rioting after Wilson shot Brown in August.

"They need to feel the pain these mothers feel at the (expletive) cemetery," shouted Paulette Wilkes, 40, a teacher's assistant who was in the crowd at the police department.

A smaller, calmer crowd of about three dozen protesters gathered outside the courthouse where the grand jury had met. In that crowd, a white woman held a sign that read: "Black Lives Matter." Many of the protesters looked stunned.

"That's just how the justice system works - the rich are up there and the poor are down here," said Antonio Burns, 25, who is black and lives in the Ferguson area. The police "think they can get away with it," Burns said.

A handful of Amnesty International volunteers in bright vests tried to maintain the peace.

'Rightfully angry'

Speaking to Al Jazeera, Dante Barry, executive director of Million Hoodies for Justice, said the protesters were "rightfully angry".

"There is this misconception that we are living in a post-racial society. Even with a black president, it [police killings of African Americans] is still happening under his watch.

"What you’re seeing tonight is folks expressing frustration and it’s rightfully so."

Some of the evidence presented to the grand jury: Michael Brown was shot 12 times by officer. The final shot fired hit the top of his head.

The results of all the autopsies on Brown's body were consistent with each other.

And the Missouri Prosecutor said some witnesses' testimony was discredited by the physical evidence available.

Al Jazeera's Tom Ackerman, reporting from Washington DC, said the evidence used by the grand jury for their decision will be published.

"Now that the case is closed, the evidence will be made public, including audio recordings and transcripts of the testimonies, which is not usual."

Brown's family quickly urged a non-violent response to the grand jury's decision. In a statement released shortly after the decision the family said: "We are profoundly disappointed that the killer of our child will not face the consequence of his actions."

"While we understand that many others share our pain, we ask that you channel your frustration in ways that will make a positive change. We need to work together to fix the system that allowed this to happen," the statement added.

"We respectfully ask that you please keep your protests peaceful. Answering violence with violence is not the appropriate reaction. Let's not just make noise, let's make a difference," it said.

Officials urged tolerance and assured residents that the National Guard would provide security at critical facilities such as fire houses, police stations and utility substations.

"I do not want people in this community to think they have to barricade their doors and take up arms," St Louis County Executive Director Charlie Dooley said before the grand jury's decision was announced.