CHICAGO - A white Chicago police officer who shot a black teenager 16 times last year was charged with first-degree murder Tuesday, hours before the city released a video of the killing that many people fear could spark unrest.

Protesters began marching through Chicago streets after the video's release. Several hundred people were blocking traffic Tuesday night on the city's near West Side. Some circled police cars in an intersection and chanted "16 shots."

The protests came after Chicago police released the video of 17-year-old Laquan McDonald's 2014 death. Prosecutors Tuesday charged Officer Jason Van Dyke with first-degree murder.

City officials and community leaders had been bracing for the release of the dash-cam video, fearing the kind of unrest that occurred in cities such as Baltimore and Ferguson, Mo., after young black men were slain by police or died in police custody.

A judge ordered that the recording be made public by Wednesday. Moments before the footage was released Tuesday evening, the mayor and the police chief appealed for calm.

"People have a right to be angry. People have a right to protest. People have a right to free speech. But they do not have a right to ... criminal acts," Police Superintendent Garry McCarthy said.

The relevant portion of the video runs for less than 40 seconds and has no audio.

McDonald swings into view on a four-lane street where police vehicles are stopped in the middle of the roadway. As he jogs down the street, he appears to pull up his pants and then slows to a brisk walk, veering away from two officers who are emerging from a vehicle and drawing their guns.

Almost immediately, one of the officers appears to fire from close range. McDonald spins around and crumples to the pavement. The second officer simultaneously lowers his weapon.

The car with the camera continues to roll forward until the officers are out of the frame. Then McDonald can be seen lying on the pavement, moving occasionally. At least two small puffs of smoke can be seen coming off his body as the officer continues firing.

In the final moments, an officer can be seen kicking something out of McDonald's hands.

Police have said the teen had a knife. Cook County State's Attorney Anita Alvarez said Tuesday that a three-inch knife with its blade folded into the handle was recovered from the scene.

City officials spent months arguing that the footage could not be made public until after the conclusion of several investigations. After the judge's order, the investigation was quickly wrapped up and a charge announced.

Alvarez defended the 13 months it took to charge Van Dyke. She said cases involving police officers present "highly complex" legal issues.

Alvarez said the impending release prompted her to move up the announcement of the murder charge out of concern that the footage could spark violence.

"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. I have absolutely no doubt that this video will tear at the hearts of all Chicagoans."