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

Meanwhile, Cleveland police and prosecutors faced continued criticism on Tuesday, a day after a grand jury decided not to charge two white police officers in the shooting death of Tamir Rice, a 12-year-old boy in 2014 who was playing with a toy gun in a park. Rice was black.

Chicago Police Officer Jason Van Dyke faces six murder charges and one count of official misconduct for killing Laquan McDonald, 17, in October 2014. Van Dyke's lawyer has said his client is seeking to have the case tried outside of Cook County.

The release last month of a video of the shooting, which shows Van Dyke shooting McDonald 16 times, set off a wave of protests and calls for Emanuel's resignation.

Van Dyke, who is white, will be assigned a judge during the hearing in the nation's third-largest city.

High-profile killings of black men by mainly white police officers since mid-2014 have triggered waves of protests across the country and fueled a civil rights movement under the name Black Lives Matter.

Over the weekend, the fatal shootings of two black people in Chicago by a police officer set off more protests, and led to Emanuel's decision to cut short his family vacation to Cuba to deal with the fallout.

Bettie Jones, 55, a mother of five, and college student Quintonio LeGrier, 19, were killed early Saturday by an officer responding to a call that LeGrier was threatening a family member with a baseball bat. Police said Jones was killed by accident during the altercation with LeGrier.

In Cleveland, some protesters took to social media on Tuesday to ask Cleveland Cavaliers basketball star LeBron James not to play to help pressure the U.S. Department of Justice to get involved in an investigation, using the hashtag #NoJusticeNoLebron. The team is scheduled to play the Denver Nuggets in Denver on Tuesday night.

Protests over the shooting of Laquan McDonald led to the resignation of Chicago's police chief and a U.S. Department of Justice probe into whether the department use lethal force too often, especially against minorities.

A protest focusing on police issues and a call for Emanuel to resign is planned at City Hall on Thursday.