By Ellen Wulfhorst and Daniel Wallis

FERGUSON, Mo. (Reuters) - Gunshots pierced the air as an angry crowd threw bottles outside the Ferguson Police Department in suburban St. Louis after a grand jury on Monday decided not to indict a white officer in the shooting death of an unarmed black teen.

The police lobbed tear gas in response as a few hundred protesters took to the streets, with some setting fire to a police cruiser and another car and some buildings.

Across the United States, the grand jury's decision sparked mainly peaceful protests as Americans spoke out on racial bias and police violence, issues so raw and emotional that they are often like a tinderbox waiting for a match.

In New York marchers chanted "Black lives matter" as they snarled traffic in Times Square. 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 by police officer Darren Wilson. In Seattle, protesters blocked a downtown street in a "die-in" protest as they lay down on the ground.

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.

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 Officer Darren Wilson.

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

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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. Brown's family 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.

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 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 police were arriving on the scene, 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.

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

(Additional reporting by Adrees Latif in Ferguson, Sascha Brodsky and Paul Thomasch in New York, Scott Malone in Boston, Fiona Ortiz in Chicago, Eric M. Johnson in Seattle; Writing by Barbara Goldberg, Eric Walsh and Peter Cooney; Editing by Paul Tait and Leslie Adler)