Laquan McDonald case: Obama 'disturbed' by dashcam video
President Barack Obama has said he is "deeply disturbed" by a police video that shows a white officer shooting a black teenager 16 times in Chicago.
Officer Jason Van Dyke is charged with the first-degree murder of 17-year-old Laquan McDonald, who was stopped by police after slashing car tyres.
In a Thanksgiving message on Facebook, the president paid tribute to Chicago protesters for keeping the peace.
Four people were arrested overnight as demonstrators marched through the city.
The protests were not as widespread or as tense as Tuesday night, which began moments after police released the dashcam video showing the incident.
Mr Obama said on Facebook: "Like many Americans, I was deeply disturbed by the footage of the fatal shooting of 17-year-old Laquan McDonald.
"This Thanksgiving, I ask everybody to keep those who've suffered tragic loss in our thoughts and prayers, and to be thankful for the overwhelming majority of men and women in uniform who protect our communities with honour."
McDonald was stopped by police in October 2014 after reports that he had been walking in the road and slashing car tyres.
The video shows him holding a knife when the police cars arrive and Mr Van Dyke emerges from his car. Within seconds, he shoots the teenager.
The officer's lawyer said he feared for his life but prosecutors said there was no justification for his actions.
According to court documents seen by the Associated Press, Mr Van Dyke was the subject of 18 civilian complaints over 14 years, including allegations that he used racial slurs and excessive force.
The death of Laquan McDonald has become the latest flashpoint in a long-running debate over police violence against black men.
Protesters said they were organising a Black Friday event which will try to shut down a major shopping area in Chicago on one of the busiest retail days of the year.
Other deaths that made headlines
Michael Brown: Police shot the unarmed teenager (above) in Ferguson last year, setting off huge protests and unrest nationwide
Eric Garner: Choked to death while under restraint by police in New York
Tamir Rice: Police shot and killed the 12-year-old in Cleveland as he brandished a toy gun
Walter Scott: A South Carolina police officer shot Scott numerous times in the back as he was running away
Freddie Gray: The 25-year-old died after sustaining a severe spinal cord injury while in the custody of police in Baltimore