FERGUSON (Reuters/AFP) - Gunshots rang out and police lobbed tear gas at an angry crowd that threw bottles outside the Ferguson Police Department in suburban St. Louis after a grand jury decided not to indict a white officer in the shooting death of an unarmed black teen. St Louis county police reported heavy automatic gunfire in one area.

Outrage over the decision fueled what had been mostly peaceful protests across the United States on Monday.

Angry people took to the streets from Seattle out west to Chicago in the middle and New York, the nation's largest city, as the hotspot town of Ferguson, Missouri - where the decision was rendered - roiled with rioting.

Not long after President Barack Obama appealed for calm over the hotly awaited ruling in Missouri in the case of slain teen Michael Brown by white policeman Darren Wilson, several hundred people gathered in front of the White House.

They chanted slogans such as "Hands up, don't shoot" and waved banners saying "stop racist police terror" and "Justice for Mike Brown". That demonstration was peaceful, unlike violence that broke out in Ferguson itself in which a police car was torched, stores were looted and police fired tear gas to disperse angry black demonstrators.

The rally outside the White House dispersed after midnight but demonstrators headed elsewhere, possibly toward the US Capitol, the seat of Congress, AFP correspondents observed.

In New York City's Times Square, several hundred people rallied carrying banners protesting "police tyranny". Others had slogans saying "racism kills" and "no justice, no peace". Some people likened the police to the white supremacist group the Ku Klux Klan. Marchers chanting "Black lives matter" snarled traffic on Broadway through Times Square.

The police presence in New York City was heavy. A helicopter flew overhead. Another rally was held in the city's Union Square.

New York City police chief Bill Bratton made an appearance at the Big Apple's signature crossroads Times Square, where someone splashed him in the face with a red liquid.

In Chicago, demonstrators walked up Lake Shore Drive carrying banners that read "Justice for Mike Brown" - the 18-year-old who was shot and killed in Ferguson on Aug 9.

Protesters in both Boston and Seattle observed the 4.5 minutes of silence that the Brown family requested after the decision was announced, with protesters in Boston then marching from City Hall to the statehouse.

In Los Angeles, at least 50 demonstrators tried to walk onto the Santa Monica Freeway from an off-ramp to block traffic, but they peacefully obeyed orders from California Highway Patrol (CHP) officers to turn back, CHP spokesman Edgar Figueroa said.

A "handful" of protesters who apparently climbed up from another direction managed to dash the freeway as the police were arriving on the scene, Mr Figueroa said. No one was injured and there were no arrests, but the freeway was shut down in both directions for about 10 minutes until the incident was over, he said.

Mr Figueroa said the freeway intrusion was an offshoot of demonstrations held in Leimert Park, a predominantly African-American neighborhood of Los Angeles.

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 the authorities said the grand jury had voted not to indict officer Darren Wilson.

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

St Louis County Prosecuting Attorney Robert McCulloch said Officer Darren Wilson had fired 12 times after getting into an "altercation" with Brown, and that the jury had found no grounds to file charges.

As Mr McCulloch rounded off his summary of the grand jury testimony, Brown's mother burst into tears and the crowd began to chant: "Hey, hey, ho, ho! These killer cops have got to go."

Members of an angry crowd outside the police station where Wilson had been based threw bottles and stones. A police car was set alight and stores looted.

Riot officers responded with teargas and flash grenades, and running battles broke out in the streets of the St Louis suburb.

Looters smashed their way into a mobile phone store opposite the police headquarters and ransacked it. Nearby, an AFP journalist was hurt when he was hit in the face by a hurled object.

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 Ms 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 Mr Antonio Burns, 25, who is black and lives in the Ferguson area. The police "think they can get away with it", Mr Burns said.

A handful of Amnesty International volunteers in bright vests tried to maintain the peace. Brown's family quickly urged a non-violent response to the grand jury's decision.

Officials urged tolerance and assured residents that the National Guard would provide security at critical facilities like 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.

President Barack Obama and the family of late 18-year-old Michael Brown separately appealed for calm after a prosecutor said a grand jury had found the policeman acted in self-defense.

Ms Pam Bailey, a retiree from St Louis in her 60s, said she had expected the decision. "I've lived long enough to know that African Americans are not considered human beings," she said.

Outside the White House, a crowd waved signs urging the government to "Stop racist police terror."

In Washington, Mr Obama made a rapidly organized televised appearance to appeal for calm in the Midwestern town, echoing the sentiments of the dead teenager's family.

"Michael Brown's parents have lost more than anyone. We should be honoring their wishes," Mr Obama said.

"I also appeal to the law enforcement officials in Ferguson and the region to show care and restraint in managing peaceful protests that may occur."

His call for calm fell on deaf ears in Ferguson, where police were pelted with bricks and bottles and responded with volleys of teargas.

"It shows that our justice system is corrupt," said a 21-year-old sales representative from Ferguson who gave his name as Josh.

"It shows that we have a long way to go and it shows today that justice hasn't been served for years and years. There's room for peaceful protests and there's room for violent protests."

Mr McCulloch told reporters the evidence presented to the jury had shown Wilson had shot as a legitimate act of self-defense during a tussle that broke out as he was responding to a robbery.

He said the "altercation" had broken out as Wilson was sitting in his patrol car and Brown was standing at the window.

A picture taken after the incident and released by the prosecutor showed Wilson with a very slight bruise to his right cheek.

"During the altercation, two shots were fired by Officer Wilson while still inside the vehicle," Mr McCulloch said.

After these shots were fired, Wilson is said to have left the car to pursue Brown, who at some point turned on him. Ten shots were fired and the young man was killed, hit six times.

In August, some early witnesses had said that Brown had his hands up and was surrendering when he was killed. But McCulloch said the physical evidence and other witnesses contradicted this account.

"Decisions on a matter as serious as charging an individual with a crime simply cannot be decided on anything less than a complete critical examination of all available evidence," he said.

In a statement, the Brown family said: "We are profoundly disappointed that the killer of our child will not face the consequence of his actions.

"We respectfully ask that you please keep your protests peaceful," the family added, calling for legal reform.

"Answering violence with violence is not the appropriate reaction."

The shooting of Brown, an 18-year-old who had planned to go to college, sparked weeks of sometimes violent protests and a nationwide debate about police tactics and race relations.

After the grand jury announcement, Attorney General Eric Holder said that a separate federal civil rights investigation into the incident and Ferguson policing in general would continue.

Ferguson's mainly African-American community of 21,000 has been on edge for days, braced for further protests should the officer not be indicted.

The mostly black suburb has an overwhelmingly white police force and residents complain of years of racial prejudice and heavy handed police tactics.

In the days leading up to the decision, Missouri's governor declared a state of emergency and called up the National Guard last week in readiness. The FBI has also deployed extra personnel.