WASHINGTON (AFP) - A white police officer who shot and killed an unarmed 19-year-old African-American man in the United States state of Wisconsin used justifiable force and will not face criminal charges, the authorities said on Tuesday.

The fatal March 6 shooting of Tony Robinson by officer Matt Kenny in the Midwestern university city of Madison touched off days of protests.

It was one of a series of recent police shootings of unarmed black men that sparked a national outcry over how officers treat minorities.

"I conclude that this tragic and unfortunate death was the result of a lawful use of deadly police force and that no charges should be brought," Dane County District Attorney Ismael Ozanne told a press conference. "My decision will not bring Tony Robinson Jr back. My decision will not end the racial disparities that exist in the justice system."

Mr Robinson's family quickly criticized the decision.

"This is politics and not justice," said his grandmother Sharon Irwin.

Family attorney Jon Loevy said while Mr Robinson's relatives supported community members who wished to protest, they did not support violence.

"It is not a situation where people should get hurt or the community should tear itself apart. That makes no sense," he said.

Dozens of young people marched in Madison after Mr Ozanne's announcement, carrying banners that read "Black Lives Matter."

Some chanted: "No justice, no peace, no racist police."

School leaders have asked parents to urge their teenagers not to go to demonstrations, after many teens were involved in violent unrest in Baltimore after the funeral of a young black man, Mr Freddie Gray, who died from injuries sustained in police custody.

Mr Ozanne said the police were called three times in five minutes to respond to an agitated, shirtless Mr Robinson who had allegedly hit a friend and random pedestrians, one in the face.

Family members who contacted the police indicated they thought Mr Robinson might have been under the influence of "some type of drugs", he said.

Another witness observed Mr Robinson speaking to his father and others who were not present, the prosecutor said.

Officer Kenny had to open an apartment door to reach Mr Robinson, he explained.

"Tony's behavior was aggressive, violent and (the officer) feared for his safety and that of others as well, and for the safety of Tony Robinson," Mr Ozanne added.

"There were seven shots fired" in the space of three seconds, the prosecutor said.

"All seven shots hit Tony Robinson, Jr. At close range."

Mr Loevy accused officer Kenny of changing his story and not following proper procedure.

"We're not going to answer questions. We have more questions than you do and we don't have answers," he said.

Ms Irwin said Mr Robinson had been "slandered from the beginning and has been set up...I miss him and really love him."

The American Civil Liberties Union voiced dismay at Mr Ozanne's decision.

"If Officer Kenny did not violate the law, then is anyone legally responsible for Mr Robinson's death?" asked ACLU of Wisconsin Executive Director Chris Ahmuty.

"Does the criminal law protect individuals like Mr. Robinson from deadly force exercised by police officers? Are police officers above the law?"

The Madison decision was being closely watched after recent unrest in Baltimore linked to Mr Gray's death that led the authorities to declare a citywide curfew and deploy the National Guard.

The US Department of Justice has launched a federal civil rights investigation into whether police in Baltimore have systematically discriminated against residents.

A similar investigation was sparked by the case of 18-year-old Michael Brown, who was fatally shot in Ferguson, Missouri last year, sparking extensive and sometimes violent protests across the nation.

The white officer who shot Brown, Darren Wilson, was not indicted.

But the Justice Department investigation found a widespread pattern of racial discrimination by Ferguson's largely white police force, and multiple violations of citizens' constitutional rights.