Police claim the 18-year-old was armed and fired at least three shots, but relatives say he only had a sandwich in his hand

A white off-duty police officer in the US city of St Louis has shot killed a black teenager who fired on him, police said, triggering protests just miles from the flashpoint suburb of Ferguson, Missouri.

The St Louis metropolitan police chief, Sam Dotson, said the officer was on patrol for a private security company late Wednesday when he engaged three men in a chase.

Dotson said the men ran away when they spotted the officer, who believed one of them was carrying a gun because of the way he was running. The officer chased the man, an altercation ensued and the man fired at the officer, the police chief said. The officer returned fire and killed the man, who was 18.

According to Dotson, ballistics evidence recovered from the scene suggested the teenager fired three shorts. The officer returned fire with 17 rounds. Dotson was unable to say why the officer fired so may shots.

In the wake of the shooting, people gathered in the area to protest against the killing. In Ferguson, a few miles away, there have been almost nightly protests in Ferguson since the fatal shooting in August of Michael Brown, a black 18-year-old, by a white police officer.

After the latest incident, some who identified themselves as relatives of the man who was shot told the St Louis Post Dispatch newspaper that he was not armed.

One woman said the victim was her cousin, Vonderrit Myers Jr.

“He was unarmed,” Teyonna Myers told the newspaper. “He had a sandwich in his hand, and they thought it was a gun. It’s like Michael Brown all over again.”

The officer, who has worked for the St Louis police department for six years, was doing a secondary job for a security company when he approached four men on the street, police said.

“As he exited the car, the gentlemen took off running. He was able to follow one of them before he lost him and then found him again as the guy jumped out of some bushes across the street,” said Police Lt Col Alfred Adkins. “The officer approached, they got into a struggle, they ended up into a gangway, at which time the young man pulled a weapon and shots were fired. The officer returned fire and unfortunately the young man was killed.”

He did not name the 32-year-old officer or the man killed.

After the shooting, a crowd of around 200 people gathered at the scene in south St Louis. Many of the protesters marched to a main road, partially blocking traffic and chanting: “Whose streets? Our streets?” as a police helicopter hovered overhead.

At one point, about a dozen people punched and kicked two occupied police vehicles, one that was marked and another that was unmarked. Demonstrators then broke the back window of a marked police vehicle.

As of early Thursday morning, none of the protesters had been arrested, Dotson said. Organisers of demonstrations in Ferguson have promised to intensify their protests over Brown’s killing if the officer who shot him does not face criminal charges. Three prominent members of the protest movement that sprung up after the deadly police shooting of Brown, who was unarmed, told a rally in New York on Tuesday night that there would be a fierce backlash if a grand jury declined to indict officer Darren Wilson.

“If they can’t serve justice in this, the people have every right to go out and express their rage in a manner that is equal to what we have suffered,” said Ashley Yates, a co-founder of Millennial Activists United, who was arrested last week while protesting in Ferguson.

Yates spoke alongside Tef Poe and Tory Russell, activists for Hands Up United and the Organisation for Black Struggle, hours after it emerged that authorities in Missouri were making plans to deal with potential riots in the event of Wilson avoiding prosecution.

More than a week of unrest followed Wilson’s fatal shooting of Brown on 9 August after the city officer stopped the teenager and a friend for jaywalking. Several witnesses have said Brown was shot while running away and when his hands were up. Police say he assaulted Wilson.

Police cracked down on the protests with armoured vehicles and dozens of armed officers in riot gear. Demonstrators were shot with teargas and rubber bullets. More than 220 people have been arrested since the protests began. A grand jury in St Louis county is now considering evidence gathered by a county police inquiry.