MINNEAPOLIS, United States - The Minneapolis police officer accused of killing a handcuffed African American man was charged with murder Friday as authorities declared a curfew after three nights of violent protests left parts of the city in flames.

Derek Chauvin, the white officer filmed kneeling on the neck of a handcuffed George Floyd for nearly nine minutes, was charged with one count of third degree murder -- unintentionally causing a death -- and one count of negligent manslaughter.

"This case is now ready, and we have charged it," said county prosecutor Mike Freeman amid outrage over the latest death of an African American in police custody.

Relatives of 46-year-old Floyd -- who spoke Friday with President Donald Trump -- welcomed news of the arrest as a "step on the road to justice," but said they hoped for tougher charges and action against the other officers involved in Floyd's detention and death.

Freeman said the three other officers were also under investigation, and that he anticipated charges. All four officers were fired from the police department Tuesday after video surfaced of Monday's arrest.

TROOPS DEPLOYED

The announcement of charges came hours after hundreds of troops were deployed to the streets of Minneapolis and St. Paul to try to prevent a fourth night of violent protests.

Scores of buildings have been burned and looted across the so-called Twin Cities, including a police station associated with the four officers.

Mayor Jacob Frey declared a night-long curfew Friday and Saturday as protesters began to appear on the streets in Minneapolis and St. Paul again Friday afternoon.

Some chanted "I can't breathe" -- Floyd's words as Chauvin's knee pressed on his neck.

"We don't care about this," said one African American demonstrator, indicating the burned-out buildings.

"If this is what it takes, we're willing to do more. You all were already killing us, when we weren't doing anything... Let's hope it makes a change," he told AFP, without giving his name.

Police were out in force in other cities as protests took place, including in New York, Washington, Atlanta, Houston and Portland, hoping to prevent recurrences of the violence that hit Minneapolis.

CHARGES CRITICIZED

Floyd's family issued a statement acknowledging Chauvin's arrest, but calling the charges against him insufficient.

"We want a first degree murder charge. And we want to see the other officers arrested," they said in a statement.

"The pain that the black community feels over this murder and what it reflects about the treatment of black people in America is raw and is spilling out onto streets across America."

The third degree charge though reflected the official autopsy saying that even though Chauvin held his knee to Floyd's neck for two minutes and 53 seconds after he became "non-responsive," Floyd did not die of asphyxiation or strangulation.

"Mr. Floyd had underlying health conditions including coronary artery disease and hypertensive heart disease," the charging document said.

"The combined effects of Mr. Floyd being restrained by the police, his underlying health conditions, and any potential intoxicants in his system likely contributed to his death."

'THIS SHOULDN'T BE NORMAL'

After beginning the day attacking the protesters as "thugs" and threatening to send in federal troops to deal harshly with them, Trump shifted tone later on Friday, announcing he had called Floyd's family to express his "sorrow."

"I understand the hurt, I understand the pain. People have really been through a lot. The family of George is entitled to justice and the people of Minnesota are entitled to live in safety," he said.

Former president Barack Obama said in a statement that he shared the "anguish" of millions of Americans over Floyd's death.

"We have to remember that for millions of Americans, being treated differently on account of race is tragically, painfully, maddeningly 'normal'," he said. "This shouldn't be 'normal' in 2020 America."

Obama's former vice president Joe Biden, who is running for president, also spoke to Floyd's family.

He called for justice and said it was time to heal the "open wound" of systemic racism in the United States.