By Mary Wisniewski and P.J. Huffstutter

CHICAGO (Reuters) - A white Chicago policeman was charged with murdering a black teenager on Tuesday, hours before authorities released a graphic video showing the youth walking away from officers as he is shot 16 times.

The footage of last year's shooting, taken from a camera mounted on the dashboard of a police car and made public under orders from a judge, prompted mostly peaceful street demonstrations in Chicago.

The clip showed 17-year-old Laquan McDonald, who authorities said was carrying a pocket knife and had the hallucinogenic drug PCP in his system, as he was gunned down in the middle of a street on Oct. 20, 2014.

McDonald is seen jogging away from patrol vehicles pursuing him from behind, and then veering off diagonally at a walk as two more officers pull up in a squad car ahead of him.

Two policemen are shown jumping out of their vehicle in the center of the road, and drawing their weapons while advancing toward McDonald, who continues to move away. Within seconds he is struck by bullets, spins and crumples to the ground, his body jerking as he is hit by additional rounds of gunfire.

McDonald's death came at a time of intense national debate over police use of deadly force, especially against minorities.

Officer Jason Van Dyke is the first Chicago police officer charged with murder for an on-duty incident in decades.

Cook County's chief prosecutor, Anita Alvarez, acknowledged that she timed the filing of the first-degree murder charge against Van Dyke to precede the video's disclosure in a bid to mute harsh public reaction to the footage.

"With release of this video it's really important for public safety that the citizens of Chicago know that this officer is being held responsible for his actions," she said.

The video originally was expected to be released a day later, in compliance with the Wednesday deadline set by a judge. However, police said they made the clip public Tuesday evening after it was leaked to a local television station. Technical issues were blamed for a lack of audio with the tape.

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Authorities, including Mayor Rahm Emanuel, appealed for calm as the city hoped to avert the turmoil over race and the use of lethal police force that has shaken much of the United States for more than a year.

Hundreds of protesters gathered after dark a few miles east of the site of the shooting. They marched through the streets chanting: "You don't get to kill us and tell us how to feel. You don't get to shoot us and tell us how to heal."

Police said two protesters were arrested after a scuffle on Michigan Avenue.

"It's a good start that they have charged the police officer, but we've clearly reached the point where something needs to change," said protester Monique Winegard, 33.

Demonstrators had assembled outside a police precinct by 9 p.m. CST (0300 GMT), demanding the release of the two people arrested. Protesters later surrounded a police vehicle in a brief standoff.

The crowd began to wane as the night wore on, and the late-autumn air grew colder, although a core group of at least 200 continued to march from block to block in what were essentially large circles enclosed by police lines. Some protesters briefly blocked an expressway entrance ramp.





'OFFICER WENT OVERBOARD'

The video, and prosecutors' account of the shooting, contradicted assertions by Van Dyke's lawyers and the police union that the shooting was justified because Van Dyke felt threatened by the youth.

Prosecutors said Van Dyke fired the shots within 30 seconds of arriving at the scene, and just six seconds after emerging from his patrol car, emptying his gun at McDonald and preparing to reload. McDonald was hit by 16 rounds, all from Van Dyke's weapon, the prosecutors added.

"Clearly, this officer went overboard and he abused his authority, and I don't think use of force was necessary," prosecutor Alvarez said at a news conference after Van Dyke's initial hearing.

Van Dyke was denied bail at a hearing in Chicago's main criminal courthouse hours after the criminal charge was announced. He could face 20 years to life in prison if convicted.

Cook County Circuit Court Associate Judge Donald Panarese scheduled a second hearing for Monday, where he said he would view the video in court and reconsider bail based on its content. Prosecutor Bill Delaney told the judge that witnesses and the video concur McDonald was not moving toward Van Dyke.





MISCONDUCT

Van Dyke has had 20 misconduct complaints made against him during the past 4-1/2 years, none of which led to any discipline from the Chicago Police Department, according to research by Craig Futterman, a University of Chicago law professor and expert on police accountability issues.

"The Chicago Police Department refuses to look at potential patterns of misconduct complaints when investigating police misconduct," Futterman said. "If the department did look at these patterns when investigating police abuse, there is a great chance right now that 17-year-old boy would still be alive."

A number of U.S. cities have seen protests over police violence in the past 18 months, some of them fueled by video of the deaths.

The uproar was a factor in the rise of the Black Lives Matter civil rights movement and has become an issue in the November 2016 U.S. presidential election campaign.

Van Dyke's lawyer Daniel Herbert said his client would prevail in court.

"This is a case that can't be tried in the streets, it can't be tried in the media, and it can't be tried on Facebook," Herbert said.





FAMILY CALLS FOR CALM

McDonald's family called for calm, as did city authorities and black community leaders.

"No one understands the anger more than us, but if you choose to speak out, we urge you to be peaceful. Don't resort to violence in Laquan's name. Let his legacy be better than that," McDonald's family said in a statement through their lawyer.

Police shootings are frequent in Chicago, the third-largest city in the United States with 2.7 million people, roughly one-third white, one-third black and one-third Hispanic.

From 2008-2014 there were an average of 17 fatal shootings by police each year, according to data from the Independent Police Review Authority, which investigates police misconduct.

Almost all shootings, fatal and non-fatal, are found to be justified.





(Reporting by Mary Wisniewski; Additional reporting by Fiona Ortiz, Justin Madden, Nick Carey and Tom Polansek in Chicago and Ben Klayman in Detroit; Writing by Grant McCool and Steve Gorman; Editing by Paul Tait and Andrew Heavens)