The officers reported they "saw a subject that they thought might have been armed around his waistband", Police Chief Fred Waller said at a press conference late Saturday.
Protesters clashed with police well into the overnight hours, four police officers were injured, and four protesters were arrested after a Chicago police officer shot and killed a man.
Larry Merritt, a fire department spokesman, says the man was shot Saturday evening in the South Shore neighborhood and was taken to Jackson Park Hospital, where he was pronounced dead.
An argument started and officers hit protesters with batons, and protesters returned punches.
The man became combative and eventually broke free from the officers, Chief Waller said.
The police recovered a semiautomatic-type weapon and magazines from the suspect.
The shooting happened about 5:30 p.m.in the 2000 block of East 71st Street, according to police and Chicago Fire Department officials. It was a chaotic scene after the shooting.
Speaking to Al Jazeera, Issa said that the tension was so high immediately after the shooting that "it was clear there was going to be a confrontation between Chicago Police and protesters at some point".
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The city erupted in protest in 2015 after the release of a video showing white police officer Jason Van Dyke shooting black 17-year-old Laquan McDonald 16 times in 2014.
After nightfall, protesters continued to mill around the neighbourhood with police occasionally chasing them away.
Three officers sustained minor injuries and there were four arrests.
"There were some members of the community who were upset, " Guglielmi told the newspaper.
He added that many in the crowd had "a false sense of what occurred".
A crowd gathered after the shooting and rumors that the man was unarmed incited the crowd, police spokesman Anthony Guglielmi told the Chicago Tribune.
The Civilian Office of Police Accountability is investigating the use of force.
This article has been updated with a Chicago activist group and local newspaper identifying the victim.