US officials have released dashboard camera video displaying a white police officer fatally shooting a black teenager 16 occasions in Chicago last year.
Officer Jason Van Dyke has been charged with first-degree murder for killing Laquan McDonald, 17. Police say he refused to drop a knife.
Mr Van Dyke stated via his lawyer and police union he had feared for his life.
Chicago Mayor Rahm Emanuel has appealed for calm in the city.
Police use of excessive force against African Americans has been the subject of nationwide protests for months.
“It is fine to be passionate but it crucial to remain peaceful,” Mayor Emanuel told a news conference.
Chicago has been preparing for attainable protests ahead of the release the police dashcam video.
In Minneapolis on Monday night, one of these protests was marred by violence when 3 white guys opened fire at a rally, injuring five individuals.
Analysis: Rajini Vaidyanathan, BBC North America correspondent
A year to the day since violent protests took location in Ferguson, Missouri, America is as soon as once again discussing the situations surrounding the fatal shooting of a black teenager by police.
It has taken campaigners in Chicago a year for police dashcam footage of the death of Laquan McDonald to be released. In Ferguson there have been no videos to show what occurred – prompting nationwide calls for physique cameras – and the debate still rages on right now about no matter whether or not the grand jury created the correct choice to acquit police officer Darren Wilson.
In this case, the officer involved, Jason Van Dyke, has now been charged with very first degree murder.
Even though several complain this has all been a long time coming, officials hope these actions will diffuse any tensions in Chicago, 1 of America’s biggest and most diverse cities.
Step-by-step guide to what occurred
In the Chicago case, a post-mortem showed Mr McDonald had been shot at least twice in the back.
State lawyer Anita Alvarez stated Mr Van Dyke had not been justified in shooting the teenager.
She mentioned Mr McDonald had been carrying a knife when he was stopped by police in 2014 but had not been threatening them or moving towards them.
Mr Van Dyke opened fire seconds after he arrived and continued shooting at the teenager as he lay on the ground, the lawyer added.
“Whilst we count on officers to do their jobs… there are a handful of negative apples who go as well far and break the law,” she mentioned.
Illinois Governor Bruce Rauner has said the video is “extremely troubling” and he expects a sturdy public reaction, although he hopes it will be “thoughtful and peaceful”.
The police union in Chicago had opposed the release of the video and said it would taint a trial jury.
Chicago police say Mr McDonald threatened officers with a knife and slashed the tyres and windows of a patrol automobile.