Protesters set buildings ablaze and looted stores in the US town of Ferguson on Monday after a grand jury chose not to press charges against a white officer who shot dead a black teen.

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

The shooting of Brown back in August sparked weeks of sometimes violent protests and a nationwide debate about forceful police tactics and race relations in America.

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 McCulloch rounded off his summary of the grand jury's decision, 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 threw bottles and stones. A police car was set alight and nearby stores looted.

Riot officers responded with teargas, batons and flash grenades, and running battles broke out in the streets of the St Louis suburb, with armored cars moving slowly through the area.

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

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.

Protest marches began in several more US cities -- including New York, Chicago and the capital Washington DC -- but there were no immediate reports of unrest outside Missouri.

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

Inside the executive mansion, 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," 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. "There's room for peaceful protests and there's room for violent protests."

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 at the window. Wilson testified to the jury that Brown leaned into the car and attacked him, grabbing his gun.

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," McCulloch said.

Twelve shots

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."

Plea for non-violence

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.