By Kenny Bahr

ST LOUIS, Mo. Oct 8 (Reuters) - A white off-duty policeman shot and killed a black teenager in St Louis on Wednesday, officers said, triggering a night of protests just miles from the site of another police shooting of another black youth in the suburb of Ferguson.

Police said the 18-year-old was armed and fired three shots while he was being chased by the officer, and they had recovered a gun at the scene.

The youth was killed almost two months to the day since sometimes violent protests erupted in Ferguson after a white police officer shot dead unarmed black 18-year-old Michael Brown.

In Wednesday's shooting, the dead man was one of three people who fled after being approached by the officer, a six-year veteran of the department who was working for a private security company, St. Louis Metropolitan Police Chief Sam Dotson said.

The officer, who was wearing his city police uniform, fired 17 shots at the teenager, police added.

A crowd of around 200 gathered at the scene in the south St. Louis neighborhood of Shaw, 11 miles (18 km) south of Ferguson. Many of the protesters marched to a major thoroughfare, partially blocking traffic and chanting "Whose streets? Our streets?" as a police helicopter hovered overhead.

Teyonna Myers, 23, told the St. Louis Post-Dispatch newspaper that she was the cousin of the suspect and that he was unarmed when he was killed.

"He had a sandwich in his hand, and they thought it was a gun. It's like Michael Brown all over again," she told the paper. Police have not named the teenager.

'MICHAEL BROWN ALL OVER AGAIN'

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.

None of the protesters, some of whom were from Ferguson, had been arrested by the early hours of Thursday, police chief Dotson told a news conference.

"I think the department showed a tremendous amount of restraint," Dotson said.

The officer, who was not hurt, has been placed on administrative leave and an investigation was under way, police said.

St Louis' historic Shaw district has a relatively low crime rate - as of September, there had been no homicides this year and just five cases of aggravated assault, according to police crime statistics.

In Ferguson, a grand jury is expected to decide next month whether to bring criminal charges against police officer Darren Wilson, who shot dead Michael Brown on Aug. 9.

Brown's death triggered weeks of sometimes violent protests, prompting the governor at one point to summon the National Guard.

Missouri authorities are drawing up contingency plans and seeking intelligence from other police departments around the country, fearing that fresh riots could erupt if a grand jury does not indict Wilson. (Reporting by Curtis Skinner in San Francisco; Editing by Dan Whitcomb and Andrew Heavens)