FERGUSON (REUTERS) - Police in Ferguson, Missouri, fired tear gas, stun grenades and smoke bombs, to disperse some 350 protesters late on Wednesday, the fourth night of racially-charged demonstrations after police shot to death an unarmed black teen.

Some demonstrators hurled rocks at police as others scattered down side streets and into cars, while smoke engulfed the area. A Reuters reporter saw two young men preparing what appeared to be petrol bombs in a bus-stop shelter, their faces covered by bandanas.

Protesters have gathered every night since Saturday when 18-year-old Michael Brown was shot to death in the mostly black suburb of St. Louis, during what authorities said was a struggle over a gun in a police car.

National figures from President Barack Obama to civil rights activist Rev. Al Sharpton have called for a peaceful response to the shooting. Obama, who is vacationing on the Massachusetts island of Martha's Vineyard, was briefed on Wednesday night about events in Ferguson by senior advisor Valerie Jarrett and US Attorney General Eric Holder, the White House said.

About 40 protesters have been arrested since Saturday. Police have sent a heavy deployment of camouflage-clad officers in body armour, including one manning a rifle on a tripod atop an armored car, to Ferguson.

"I've had enough of being pushed around because of the color of my skin. I'm sick of this police brutality," said one protester who only gave his first name, Terrell, 18. "I'm going to keep coming back here night after night until we get justice," he said.

Police have been slow to release information about the shooting, except to say that it followed a struggle between the unnamed officer and Brown, and that the officer was treated at a hospital for swelling on the side of his face.

One witness who was walking with Brown at the time has said in media interviews that Brown put his hands in the air and was not struggling with the officer. He said the officer fired multiple times into Brown's head and chest.

The witness, Dorian Johnson, was expected to meet on Wednesday with prosecutors and investigators, local media reported. His lawyer, Freeman Bosley, a former St. Louis mayor, did not immediately answer requests for comment.

Another witness, Tiffany Mitchell, told CNN she watched Brown and the officer "tussling through the window," with the officer pulling the teen in as he struggled to get away, and then a "shot was fired through the window."

"The kid he finally gets away and he starts running. As he runs, the police get out of his vehicle and he follows behind him shooting," Mitchell said, adding that Brown turned around and put his hands in the air.

"And the cop continued to fire until he just dropped down to the ground and his face just smacks the concrete."

Police have declined to release the name of the officer involved in the incident, citing concerns for his safety, a decision that has been criticised by demonstrators. The officer has been placed on administrative leave.

Police Chief Thomas Jackson told reporters his priority was improving race relations in Ferguson, which has seen a stark demographic shift in recent decades, going from mostly white to mostly black. About two-thirds of the town's 21,000-strong population are black. On a police force of 53, three officers are black.

"This is an opportunity to fix what's wrong," he said.

The local Ferguson-Florissant school district meanwhile postponed its first day of classes until Monday, cancelling classes for this Thursday and Friday. As the protest swelled on Wednesday night, journalists Wesley Lowery of the Washington Post and Ryan J. Reilly of Huffington Post were briefly arrested inside a fast-food restaurant.

Meanwhile, police in California were investigating a separate incident of an officer fatally shooting an unarmed 25-year-old black man in Los Angeles. On social media, groups claiming to be associated with the Anonymous hacker activist group called for nationwide protests and threatened to reveal personal information about Ferguson police officers.

The Ferguson police said there have been attempts to hack their website and that it was temporarily disrupted, and that the cyber-threats prompted the decision not to release the officer's name.