MISSOURI (REUTERS) - A white policeman shot dead a black man brandishing a pistol at a suburban St Louis gasoline station overnight, police said on Wednesday, igniting violence reminiscent of riots over the police killing of an unarmed black teenager in nearby Ferguson.

Black officials in Missouri were at pains to distinguish the death of a suspect with a gun from cases where unarmed black men were killed by police officers, incidents that led to protests across the United States and bitter debate about how American police forces treat non-white citizens.

The shooting happened late on Tuesday at a Mobil On The Run gas station in Berkeley, Missouri, within walking distance of the Ferguson street on which a white police officer shot dead 18-year-old Michael Brown in August.

"This is not a policeman in the city of Berkeley going out half-cocked," Berkeley Mayor Theodore Hoskins said at a news conference.

"You could not even compare this with Ferguson."

A crowd of 200 to 300 people gathered at the gasoline station after the shooting, and bricks and three fireworks were thrown, two of them at the roughly 50 officers at the scene, St Louis County police chief Jon Belmar said.

Two officers were injured and four people were arrested for assault before calm was restored, Belmar said.

The shooting occurred three days after the worst fears of police-reform activists and the direst warnings of police leaders came to pass: on Saturday afternoon, a man summarily shot dead two officers in their patrol car in New York City, targeting them only because of the uniform they were wearing.

The Berkeley encounter unfolded after the officer, a six-year veteran of the town's police department who was responding to a report of a theft, got out of his car to talk to two men at the gasoline station.

One of them pointed a loaded 9mm handgun at the officer, Belmar said.

Police released a somewhat indistinct, distant surveillance video from the gas station, edited to end just before the shooting.

In the corner of the frame, one of the people at the station can be seen shifting into what may be a shooter's stance near the police car.

Pulling out his own gun, the officer stepped backward, stumbled, and fired three shots, Belmar said.

One bullet struck the man with the gun, a second hit a police car tyre, and the third cannot be accounted for, he said.

Paramedics declared the man dead at the scene.

SECOND MAN SOUGHT

Hoskins, Berkeley's mayor, said he did not want to jump to conclusions while local and county officials investigated the shooting, but he also emphasised what he said were clear differences with the Brown case.

"We have a majority of black officers in our city," Hoskins said, drawing contrasts with Ferguson.

"The mayor is black. The city manager is black. The finance director is black. The police chief is black. Our police officers are more sensitive."

Maria Chappelle-Nadal, a black Missouri state senator who has been tear-gassed while protesting Brown's death, said she believed the Berkeley police officer was justified.

"This is a different narrative in that he had a gun," she told CNN, referring to the man who was killed.

Authorities did not identify the dead man, but the St Louis Post-Dispatch named him as Antonio Martin, 18.

The officer, who was not identified but has been put on administrative leave, had been given a body camera but was not wearing it at the time of the shooting.

The dashboard camera on the officer's car was also off.

Protests in Ferguson have taken place for months and spilled over into violence when a grand jury decided a month ago not to charge the police officer who shot Brown.

Demonstrations in cities across the country gained in momentum when a New York grand jury decided not to indict a police officer over the death of Eric Garner, a 43-year-old black man who died in July when police tackled him and put him in a choke hold.

Since the slaying on Saturday of the two policemen in New York, the NYPD said on Wednesday it had investigated about 40 threats of violence against officers and arrested four people.

In Los Angeles, police said they would investigate whether any officers were involved in the singing of a song, at a party organised by a retired policeman, that poked fun at the Ferguson killing.