A white Chicago policeman who shot and killed a black teenager has been charged with murder in a prosecution hastened in hopes of averting renewed racial turmoil over the use of lethal police force that has shaken the United States for more than a year.

A highly anticipated video of the 2014 shooting, taken from a camera mounted on the dashboard of a police car, was released hours after the officer in question, Jason Van Dyke, made his first appearance in court.

Amid intense interest from the media and public, the police department website where the footage was posted became overwhelmed and initially crashed.

The city is bracing for demonstrations, but so far only around 100 people are protesting south of Chicago's business district.

The video shows 17-year-old Laquan McDonald, who authorities say was carrying a knife and on a hallucinogenic drug, as he was gunned down in the middle of a street on October 20, 2014, as walked away from officers who confronted him.

It shows two police officers jumping from their car and drawing their weapons while advancing toward McDonald. Within seconds he is struck by bullets and crumples to the ground, his body jerking as he is hit by additional rounds of gunfire.

Prosecutors say he was shot 16 times by Van Dyke, who emptied his gun and was preparing to reload his weapon. Through his lawyer and the police union, Van Dyke has said the shooting was justified because he felt threatened by McDonald.

"Clearly, this officer went overboard and he abused his authority, and I don't think use of force was necessary," Cook County prosecutor Anita Alvarez said.

Chicago Mayor Rahm Emanuel has appealed for calm as the city prepares for possible protests.

"It is fine to be passionate but it essential to remain peaceful," Emanuel said.

Police Superintendent Garry McCarthy says police will "facilitate" protests but won't tolerate criminal behaviour.

Van Dyke was denied bail in Chicago's main criminal courthouse after prosecutors announced charges of first-degree murder. If convicted, Van Dyke could face 20 years to life in prison.

A second hearing will be held on Monday where the video will be viewed in court and bail for Van Dyke will be reconsidered.