Chicago's mayor has appealed for calm after a "chilling" video was released of a white policeman shooting dead a black teenager.
Laquan McDonald was shot 16 times in October 2014 while running down a Chicago street.
Police say the 17-year-old refused to drop a knife which was recovered from the scene and Chicago police officer Jason Van Dyke has been charged with first-degree murder.
Van Dyke was the only officer present at the scene who opened fire. Prosecutors say the officer emptied his 9mm pistol of all 16 rounds and that he was on the scene for just 30 seconds before he started shooting.
It is alleged he opened fire just six seconds after getting out of his vehicle and kept firing even though Mr McDonald dropped to the ground after the initial shots.
Community leaders have been concerned the release of the film would lead to disorder similar to that which occurred in Baltimore and Ferguson, Missouri, after black men were killed by police or died in police custody.
Groups of protesters have begun marching through Chicago following the release of the video, which was uploaded to YouTube by Chicago police (warning: link contains images some viewers may find disturbing).
Several hundred people have blocked traffic on the city's West Side while some circled police cars in an intersection and chanted "16 shots".
"I understand people will be upset and will want to protest when they see this video," said Chicago's Mayor Rahm Emanuel. "It is fine to be passionate, but it is essential that it remains peaceful."
Chicago Mayor Rahm Emanuel, left, and Police Superintendent Garry McCarthy appear at a news conference announcing first-degree murder charges against police officer Jason Van Dyke. Image: Charles Rex Arbogast / AP/Press Association Images
Police Superintendent Garry McCarthy said: "People have a right to be angry. People have a right to protest. People have a right to free speech. But they do not have a right to ... criminal acts."
City officials spent months arguing the footage could not be made public until the killing had been fully investigated.
After a judge ordered the video's release, the enquiry was suddenly wrapped up and Van Dyke was charged.
State Attorney Anita Alvarez defended the 13 months it took to charge Van Dyke and acknowledged the footage could lead to violence.
"It is graphic. It is violent. It is chilling," she said. "To watch a 17-year-old young man die in such a violent manner is deeply disturbing. I have absolutely no doubt that this video will tear at the hearts of all Chicagoans."
But some community leaders suggested Ms Alvarez only brought the charge because of the order to release the video.
"This is a panicky reaction to an institutional crisis within the criminal-justice system," said the Rev Jesse Jackson, who said he hoped to see "massive" but peaceful demonstrations.
