US officials have released dashboard camera video showing a white police officer shooting a black teenager 16 times 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 said through his lawyer and police union he had feared for his life.
Chicago Mayor Rahm Emanuel has appealed for calm in the city.
After the video was released, several hundred people held a march in Chicago, blocking traffic. During a tense but largely peaceful demonstration the crowd chanted "16 shots".