The Chicago police officer who shot a black teenager last year pleaded not guilty to murder on Tuesday, as Mayor Rahm Emanuel cut short a vacation to return to the city to deal with the fallout from two more fatal police shootings over the weekend.

Meanwhile, protesters demonstrated outside the Cuyahoga County prosecutor's office in Cleveland, a day after a grand jury decided not to charge two white police officers in the 2014 shooting death of Tamir Rice, a black 12-year-old boy who was playing in a park with a toy gun that shoots plastic pellets.

Tensions over race and policing in Chicago and Cleveland come amid intense scrutiny of police killings in the United States over the past 18 months, especially of black men.

Protests have taken place around the country and the issue has fueled a civil rights movement under the name Black Lives Matter.

Chicago Police Officer Jason Van Dyke, who is white, faces six counts of first-degree murder and one count of official misconduct for killing Laquan McDonald, 17, in October 2014.

He pleaded not guilty to all charges at the Cook County criminal court in Chicago.