FERGUSON, Mo. -- A black teenager shot multiple times by a suburban St. Louis police officer was unarmed when he died, police said Sunday, as hundreds of local residents protested and a civil rights leader expressed outrage at the killing.

Michael Brown, 18, had graduated from high school and was about to enter a local college, said his mother, Lesley McSpadden.

St. Louis County Police Chief Jon Belmar said the shooting occurred after an officer encountered two people -- one of whom was Brown -- on the street near an apartment complex Saturday afternoon in Ferguson, a predominantly black suburb a few miles (kilometres) north of downtown St. Louis.

Belmar said one of the men pushed the officer back into his squad car and a struggle began. Belmar said at least one shot was fired from the officer's gun inside the police car. Ferguson Police Chief Tom Jackson said authorities were still sorting out what happened inside the police car. It was not clear if Brown was the man who struggled with the officer.

The struggle spilled out into the street, where Brown was shot multiple times. Belmar said the exact number of shots wasn't known, but "it was more than just a couple." He also said all shell casings found at the scene matched the officer's gun. Police are still investigating why the officer shot Brown, who police have confirmed was unarmed.

Jackson said the second person has not been arrested or charged and was expected to be interviewed later Sunday. Authorities aren't sure if that person was unarmed, Jackson said.

McSpadden said she doesn't understand why police didn't subdue her son with a club or Taser instead of shooting him, and she said the officer involved should be fired and prosecuted.

"I would like to see him go to jail with the death penalty," she said Sunday at the site of the shooting, fighting back tears.

The killing drew criticism from some civil rights leaders, and they referred to the 2012 racially charged shooting of 17-year-old Trayvon Martin by a Florida neighbourhood watch volunteer who was acquitted of murder charges, as well as the New York City man who died from a police chokehold.

"We're outraged because yet again a young African-American man has been killed by law enforcement," said John Gaskin, who serves on both the St. Louis County and national boards of directors for the NAACP, a leading civil rights organization.

The Rev. Al Sharpton called the shooting death "very disturbing" and the New York-based civil rights leader said he planned to go to Ferguson to meet with the family Monday night or Tuesday.

A few hundred protesters gathered outside Ferguson Police headquarters about the time the police news conference was to begin. At one point, many of them marched into an adjacent police building, some chanting "Don't shoot me" while holding their hands in the air. Officers stood at the top of a staircase, but didn't use force; the crowd eventually left.

Protesters outside chanted slogans -- "No justice, no peace" and "We want answers" -- and some carried signs that read "Stop police terrorism" and "Disarm the police."

Later, a few thousand people crammed the street where Brown was shot for a candlelight vigil.

Critics have contended that police in the St. Louis area too often target young black men. Statistics on police-involved shootings in the region were not immediately available.

The St. Louis County Police Department is in charge of the investigation. County Executive Charlie Dooley, who showed up at the protest Sunday to urge calm, said he will request an FBI investigation. U.S. Justice Department spokeswoman Dena Iverson said Sunday that Attorney General Eric Holder had instructed attorneys in the department's civil rights division to monitor developments.

The race of the police officer involved in the shooting has not been disclosed. He has been with the Ferguson Police Department for six years, Belmar said, noting he wasn't aware of other issues involving the officer. The officer has been placed on paid administrative leave, which is a common procedure after police shootings.

Several protesters were angry that Brown's body remained on the street for hours after the killing. Belmar said that officers "had to practice our due diligence and that's why it took as long as it did."