A white Chicago policeman has been found guilty of the second-degree murder of black teenager Laquan McDonald - the first time in half a century that a Chicago police officer has been convicted for an on-duty killing.

Jason Van Dyke, 40, faces up to 20 years in prison after a jury found him guilty of second-degree murder - roughly the American equivalent of manslaughter - instead of first-degree murder. First-degree murder would have carried a life sentence, but the jury found instead that he believed his life was in danger, although that belief was unreasonable.

The Chicago police department was braced for protests after the verdict, and had in advance cancelled days off and put officers on 12-hour shifts. An extra 4,000 officers are on the street, according to spokesman Anthony Guglielmi, as activists have been planning how they might react to a verdict.

He was also convicted of 16 counts of aggravated battery, and acquitted of one count of official misconduct.

Van Dyke shot 17-year-old McDonald dead in October 2014, firing 16 bullets in 13 seconds after McDonald refused to drop a three-inch knife.

Dashcam footage of the incident was released in November 2015, sparking protests across the city. The footage showed McDonald walking away from the officers rather than charging at them, as the Chicago police department had claimed.