A crowd of about 100 people were gathered early today at the scene in Berkeley, Missouri, close to the town of Ferguson, where 18-year-old Michael Brown died in August.
Authorities did not immediately identify the man who was shot. But the St Louis Post-Dispatch reported that a woman at the scene, Toni Martin, said he was her son, 18-year-old Antonio Martin.
One of the men pulled a handgun and pointed it at the officer, Schellman said. The officer fired several shots, striking and fatally wounding the man.
The second man fled, and the dead man's handgun has been recovered, according to Schellman.
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The St Louis County Police Department is handling the investigation, and no further details about the incident were immediately available.
The protesters who gathered early today milled around the gas pumps at the station, some taunting and yelling at police officers.
Some had strands of yellow police-line tape draped around their neck, with others using it as a headband.
Authorities from multiple agencies, some in riot gear, stood among the protesters.
Across the street, another gas station's glass doors were shattered, and police were standing outside the door, turning people away.
Orlando Brown, 36, of nearby St Charles was among the protesters. He said he didn't have all the details about the shooting but said he wondered if it was a case of police aggression.
Brown said he was pepper-sprayed during the protest as police tried to separate him from a friend whose hand he was holding. He said his friend was arrested for failing to disperse.
Neither Schellman nor Berkeley police could immediately confirm that pepper spray was used or that arrests were made. Photos from the scene showed authorities scuffling with at least a few protesters.