Chicago, Illinois - A judge in the United States has sentenced former white police officer Jason Van Dyke to six years and nine months in prison, plus two years probation, for the 2014 murder of black teenager Laquan McDonald.

It is the first time in decades that a police officer in the racially divided city of Chicago has been convicted of on-duty murder and sentenced to jail time.

But activists decried Friday's sentence as too lenient, while the convicted killer said he "felt great" after the sentencing, according to his lawyer, who all spoke to the press after the emotional, nearly nine-hour hearing.

Van Dyke had pumped 16 shots into 17-year-old McDonald on October 20, 2014, continuing to fire after the boy's body hit the pavement.

Van Dyke and other officers first claimed he acted in self-defence, but late last year a mostly white jury found otherwise after viewing dashboard camera footage of the incident, which showed McDonald holding a small knife while veering away from the officer.

The footage had been withheld for more than a year after the shooting and was only released after a Freedom of Information Act request.

Once made public, the graphic video sparked outrage and protests in Chicago, leading to the firing of the city's top cop and the Federal Department of Justice launching an investigation into the Chicago Police Department.

Van Dyke faced a sentence anywhere from probation without jail time to dozens of years behind bars, but on Friday, Judge Vincent Gaughan said he would only sentence the ex-policeman for murder since it is a more serious offence than aggravated battery.

'Sad day for America'

Murder convicts in Illinois may only serve half their sentences, so Van Dyke, who has already served three months, may walk free in just over three years.

"This is a slap in the face for us, and a slap on the wrist for him," William Calloway, an activist who helped file the Freedom of Information Act request, told press outside the court. "We're devastated. We're heartbroken."

In his explanation of the sentence, Judge Gaughan focussed on the effect of the crime and trial on the families of Van Dyke and McDonald.

"This is not pleasant, and this is not easy," he said. "This a tragedy for both sides."

McDonald's great-uncle Rev Marvin Hunter told reporters after the hearing that it was a "sad day for America," while noting the significance of a Chicago police officer going to jail.

"African American people are still being treated as second class citizens when it comes to sentencing laws," he said, calling for changes in laws for harsher sentencing of officers. "But I want to say to everyone that if they sentenced him to one minute, it is a victory."

Defence lawyer Dan Herbert said the sentence was a "huge relief" for his client.

"He truly felt great [after the sentencing]," he said. "It was the first time I've seen the guy, since this whole ordeal started, that he was happy."

Different pictures

Witnesses at Friday's marathon hearing painted two very different pictures of Van Dyke.

Hunter described the ex-policeman as having "cold, callous disregard for the life of a young black man", while four other black men took the witness stand and accused Van Dyke of a pattern of violence.

One said Van Dyke held a gun against his temple and called him the N-word, another said the officer choked him in the back of a squad car, while another recounted being handcuffed and dragged by Van Dyke so badly that he required surgery and received hundreds of thousands of dollars in damages.

But defence witnesses sought to portray Van Dyke as a family man, not a serially violent, racist officer. Retired officer Kenneth Watt said Van Dyke was a "darn nice guy" and a "good police officer".

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Van Dyke's wife Tiffany repeatedly broke down as she pleaded for leniency, describing how their two school-age daughters have been bullied and struggle to sleep without their father home.

"He is my everything. He is my other half. He is a kind gentleman," she said, as her husband, wearing a scruffy beard and a yellow prison uniform, sat across the court and looked down at the table in front of him. "He is not a murderer."

Van Dyke himself made a short statement, delivered in muted tones, a sharp contrast to the defiant, sharp responses he gave to prosecutors during the jury trial.

"I pray daily for the soul of Laquan McDonald, and it was due to my actions that his family suffered the pain of the loss of a family member," he said. "I will have to live with this for the rest of my life."

Three other officers were separately charged in covering up Van Dyke's actions, but a judge acquitted them on Thursday.