CHICAGO (CBS) — The city on Tuesday released a graphic, disturbing video of a Chicago police officer fatally shooting a teenager, unloading his gun 16 times in the span of 15 seconds as the young man walked away from the officers during an altercation last year.

CBS 2 aired a portion of the video that is appropriate for television, showing the initial shot from Officer Jason Van Dyke. At this time, cbschicago.com will not provide the full video.

Mayor Rahm Emanuel and Police Supt. Garry McCarthy held a news conference, where they appealed for calm.

“People have the right to free speech,” McCarthy said. “They do not have the right to commit criminal acts.”

Officer Jason Van Dyke, “violated basic moral standards that bind our community together” when he shot 17-year-old Laquan McDonald, Emanuel said.

“Anyone there to uphold the law cannot act like they are above the law,” Emanuel said.

The mayor urged the city to heed to wishes of McDonald’s family and not resort to violence seen in other cities faced with police violence.

“I understand people will be upset and want to protest,” he said. “For those who choose to speak out, do it peacefully.”

At the press conference, there was a noted absence, the independent journalist Brandon Smith who fought the city for the video’s release

He says he was excluded, but tweeted CBS 2’s Brad Edwards a question he wanted to ask: “Will you move to reform the wider culture of policing that has led to 300 shootings in five years?”

In response, Supt. McCarthy said, “Since I got here, I’ve been talking about changing the culture of the Chicago Police Department in a positive fashion. There’s some very good things about the Chicago Police Department that I found when we got here. We’re building on those and we’re trying to revamp anything that’s negative.”

Outside public safety headquarters, CBS 2 saw Congressman Bobby Rush, who hadn’t yet seen the video. He hadn’t seen the video and then watched with us.

“I’m shocked, but I’m not shaken,” Rush said. “You have another instance of watching police murder in this city. There’s a pattern to this murder and somebody’s got to be held responsible.”

The initial protests downtown were peaceful. Later in the evening there were a few tense scuffles with police, near Michigan and Balbo.

“People of good faith love their city,” Emanuel said, adding the shooting “doesn’t speak to who we are.”

In the video, Van Dyke exits his car with his gun drawn and approaches McDonald. His first shot spins McDonald around and he collapses in the middle of Pulaski Road. The video shows McDonald walking away from the officer as Van Dyke moved closer before shooting.

Once McDonald is down, the shooting continues. After the initial shot, the video doesn’t show Van Dyke actually firing, but puffs of smoke rise from McDonald’s body, evidence of several more shots. McDonald is seen lying on his side in the road, twitching from the impact of the bullets.

After Van Dyke stopped shooting, his partner is seen kicking a knife away from McDonald’s body.

In the autopsy report, the medical examiner said the teenager suffered 16 gunshot wounds.

McDonald was armed with a knife and reportedly under the influence of PCP, but did not appear to be a direct threat to any of the eight officers on the scene. Only Van Dyke opened fire.

A judge last week ordered that the video be made public by Wednesday.

Also on Tuesday, officer Van Dyke was charged with first-degree murder for shooting Laquan McDonald in October, 2014. Van Dyke is no longer on the department payroll.

Earlier Tuesday, Cook County State’s Attorney Anita Alvarez said Van Dyke was on the scene less than 30 seconds before he started shooting, and had been outside of his squad car less than six seconds before he fired the first shot. He fired 16 shots in the span of about 15 seconds.

She said she had seen the video of the shooting several times.

“It is graphic, it is violent, it is chilling,” she said. “To watch a 17-year-old young man die in such a violent manner is deeply disturbing, and I have absolutely no doubt that this video will tear at the hearts of all Chicagoans.”

Van Dyke, a 14-year veteran of the force, was one of eight officers responding to calls of a suspected thief armed with a knife, but no other officer saw a need to use force that night, prosecutors said.

Alvarez said there also were several civilians who witnessed the shooting, including a motorist stopped on Pulaski Road, who said it appeared McDonald was trying to get away from police at the time.

“The motorist stated that McDonald never moved toward, launched at, or did anything threatening towards the officers before he was shot, and fell to the ground,” Alvarez said.