A video feed showed the petrol station cordoned off by yellow tape and guarded by police, some in helmets and carrying riot shields, with bystanders shouting at them in a tense stand-off.

The shooting of Brown earlier this year, and the decision not to prosecute the officer involved, set off demonstrations across the United States. The St Louis Post-Dispatch newspaper reported about 60 people had gathered at the scene of the incident and that at least three were arrested.

Images and video footage showed a flash, a loud bang and smoke filling an area near the petrol pumps, but it was not clear whether they were caused by bystanders or the police. Local broadcaster KSDK reported people hurled rocks and bricks toward police.

Police said the man who was killed had pointed a handgun at an officer who was conducting a "routine business check" and had approached two men outside the petrol station after 11pm on Tuesday (4pm on Wednesday AEDT) in the suburb of Berkeley.

"Fearing for his life, the Berkeley officer fired several shots, striking the subject, fatally wounding him," St. Louis County Police Department spokesman Brian Schellman said. The second man fled the scene.

The St Louis Post-Dispatch named the dead man as 18-year-old Antonio Martin, citing his mother, who said he was with his girlfriend around the time of the shooting.

"They won't tell me nothing. His girlfriend told me that the police was messing with him," the man's mother, Toni Martin, told a local broadcaster. "When he was trying to get up and run, they start shooting."

St. Louis County police recovered the deceased's man's handgun at the scene. They did not confirm his identity.

Dead man's family in shock

Antonio Martin's extended family was in shock early Wednesday, as they waited for details to unfold about the fatal shooting of the 18-year-old at a Berkeley gas station.

"This doesn't make any sense for them to kill my son like this," Toni Martin-Green said early on Wednesday from her home located near the University of Missouri-St Louis campus. "I am trying to be calm."

She said initial video provided by police didn't offer enough information to answer her questions.

"He's like any other kid who had dreams or hopes," Green said. "We loved being around him. He'd push a smile out of you."

Green described his son as a "follower" who took medication for being hyperactive.

"It was hard for him to focus," Green said.

Martin mainly grew up in the Hyde Park area of North St Louis before moving with family to unincorporated St Louis County a few years ago. Martin attended high school in Jennings before dropping out and had also been enrolled in the federal Job Corps program for a spell. He last worked at White Castle and wanted to go back to Job Corps, his father said.

Police say the man shot had a criminal record, with charges including three assaults, armed robbery, armed criminal action and multiple uses of weapons since he was 17.

Martin's parents acknowledged that their son has been arrested before and had "stumbled in the past."

"In the last year, he was really trying to find who he was. He was ready to take the world on," the father said. "He knew he had parents who loved him. He had that support."

"He was not a violent person, to our knowledge," he added. "Around us there weren't any pistols. It's hard to believe that."

His grandmother, Margret Chandler, was also in disbelief.

"When he was around me, he knew to do right," she said. "Why would he pull out a gun against the police? That's the thing I don't get. It just doesn't add up."

African-American mayor defends policeman  

"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."

Hoskins said he did not want to jump to conclusions, but 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.

Civil rights protests continue

Berkeley neighbours the suburb of Ferguson, where police officer Darren Wilson shot Michael Brown on August 9, a killing that fuelled criticism of the way police and the criminal justice system treat minority groups.

Protests in Ferguson have taken place for months and spilled over into violence when a grand jury decided not to charge Wilson.

Demonstrations in cities across the country gained in momentum when a New York grand jury decided not to charge police over the death of Eric Garner, a 43-year-old black man whom police tackled and put in a chokehold.

Before the latest incident, about 200 people marched in New York on Tuesday, defying Mayor Bill de Blasio's call for protests to be suspended after two police officers were killed in their patrol car on Saturday in an apparent revenge attack.

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.

The lyrics of the song, on a video posted on entertainment news website TMZ, said: "Michael Brown learned a lesson about a messin' with a badass policeman."

Reuters