It started with silence as the crowd strained to listen to the prosecutor over the radio. Then tears gushed from the eyes of Michael Brown's mother.

No indictment. The white police officer who shot dead her unarmed 18-year-old son more than three months ago in the predominantly African American suburb of Ferguson outside St. Louis, Missouri would not face trial.

Within minutes, blind rage would take hold. Agitators went on the rampage, smashing windows, looting stores, attacking a police car and torching buildings.

But first, supporters hugged mother Lesley McSpadden, taking her into their arms.

"We're with you. We love you, we support you," murmured the crowd watching her break down.

"Defend himself from what? What was he defending himself from? Tell me that.

"Everybody wants me to be calm. Do you know how them bullets hit my son? What they did to his body? Ain't nobody had to live through what I had to live through," she said.

"Why, oh why? Why?" she cried.

Brown's family released a statement saying they were "profoundly disappointed" in the decision but asked the public to "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".

US President Barack Obama echoed Brown's parents in calling for any protests to be peaceful.

"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 on Monday night from the White House.

Resignation, disappointment, shock and anger took hold. The protest attended by several hundred outside the Ferguson police department degenerated into chaos.

"No justice, no peace," they shouted. "Hey, hey, ho, ho these killer cops have got to go."

The tension built quickly. Agitators wove through the crowd, aggressive and angry, taunting police, screaming at protesters being interviewed by TV cameras.

At least a dozen buildings were set on fire in Ferguson, and St. Louis County Police Chief Jon Belmar said he personally had heard about 150 gunshots fired during a night of looting, arson and clashes between demonstrators and police that resulted in at least 29 people arrested.

Protesters lobbed glass bottles as police in full riot gear huddled behind their cordon at the police station, initially impervious to the chants.

"You need to stop throwing rocks immediately at our police or you will be subject to arrest," an officer shouted repeatedly through a loudspeaker.

Then came the tear gas. Protesters fled. Others egged on the police. Journalists put on their gas masks.

Despite the chaos, peaceful protesters young and old stood on the sidewalk, determined to stand their ground.

It was about more than the death of an 18-year-old high school graduate who was preparing to go to technical college, they said.

"This is something they've always done. I'm 63 years old. I have seen this back in the days with Martin Luther King. They never change, and they ain't going to never change," said one man who declined to give his name.

Thousands protested from Los Angeles to New York, leading marches, waving signs and shouting chants of "Hands Up! Don't Shoot," the slogan that has become a rallying cry in protests over police killings.

AFP - AP - Reuters