Former US president Barack Obama believes the protests following the death of George Floyd in police custody have provided the opportunity for people to be "awakened" and "change America".

His first remarks on the country’s unrest come as the release of Mr Floyd’s autopsy shows he tested positive for COVID-19 more than six weeks before his death.

Mr Obama struck a note of optimism, while acknowledging the despair and anger powering the protests since the death of George Floyd in police custody nine days ago.

"In some ways, as tragic as these last few weeks have been, as difficult and scary and uncertain as they’ve been, they’ve also been an incredible opportunity for people to be awakened to some of these underlying trends," Mr Obama said via livestream from Washington.

"They offer an opportunity for us to all work together to tackle them, to take them on. To change America and make it live up to its highest ideals.

"Part of what’s made me so hopeful is the fact that so many young people have been galvanised and activated and motivated and mobilised."

Mr Obama’s speech offered a contrast in tone to the way his successor, Republican President Donald Trump, has responded to the protests, some of which have devolved into violence.

Mr Floyd, meanwhile, tested positive for COVID19 on April 3 but was not showing symptoms, according to a 20-page report released by the Hennepin County Medical Examiner’s Office.

On Monday (local time) the coroner’s office said Mr Floyd had a heart attack while being restrained by officers, and classified his death as a homicide.

The latest report by Chief Medical Examiner Andrew Baker noted that Mr Floyd’s lungs appeared healthy, but he had some narrowing of arteries in the heart.

The county’s earlier summary report listed fentanyl intoxication and recent methamphetamine use under "other significant conditions" but not under "cause of death".

The full report’s footnotes noted that signs of fentanyl toxicity can include "severe respiratory depression" and seizures.

Earlier, prosecutors levelled new criminal charges against four Minneapolis police officers over the death of Mr Floyd, who was pinned by his neck to the ground during an arrest that has sparked more than a week of nationwide protest.

The added murder charge filed against one officer already in custody and the arrest of three more accused of playing a role in the killing of Mr Floyd, a 46-year-old African American, came as several nights of escalating unrest gave way to mostly peaceful protests.

Thousands of demonstrators later massed near the White House and lit up their mobile phone flashlights and sang along to the 1970s soul tune Lean on Me, before resuming a chorus of anti-police chants.

In a further display of self-policing seen in Washington and elsewhere this week, protesters urged some of their more provocative cohorts to stop taunting police and leave.

Several major cities scaled back or lifted curfews but not all was calm.

One New York police officer was shot shortly before midnight Wednesday and another officer was slashed in Brooklyn, police said, but it was unclear if the attack was related to ongoing protests.

The names and conditions of the officers were not released. Both were injured near Church and Flatbush avenues in Brooklyn and were taken to King’s County Hospital, police said.

One arrest was made at the scene.

New York Police Department Chief of Department Terence Monahan has spoken out about the arrests in New York City.

"When we have these big crowds, especially in this area, especially where we’ve had the looting, no more tolerance. They have to be off the street. An 8 o’clock curfew, we gave them until 9 o’clock, and there was no indication that they were going to leave these streets," Mr Monahan said.

Protesters march in Manhattan over the killing of George Floyd. Photo: Getty

City Public Advocate Jumaane Williams, who attended a rally in downtown Brooklyn, expressed outrage that peaceful demonstrations were broken up.

"I can’t believe what I just witnessed & experienced," Mr Williams wrote on Twitter. He called the use of force on nonviolent protesters "disgusting."

The Associated Press is reporting that roving bands of looters that moved through the city on Sunday and Monday nights were nowhere to be seen, and the protesters who were arrested were "orderly".

In New York City’s Brooklyn borough, police in riot gear charged into about 1000 protesters defying a curfew, albeit peacefully, near a plaza, and clubbed demonstrators and journalists as they scurried for cover in heavy rain.

The confrontation in Brooklyn seemed to be the biggest exception to a calmer night, hours after the new charges in Minneapolis.

Derek Chauvin, jailed on Friday on charges of third-degree murder and manslaughter, was newly charged with second-degree murder.

The added charge, defined under Minnesota law as unintentionally causing another person’s death in the commission of a felony offence, can carry a sentence of up to 40 years, 15 years longer than the maximum for third-degree murder.

Mr Chauvin, 44, was the white officer seen in widely circulated video footage kneeling on Mr Floyd’s neck for nearly nine minutes as the latter gasped for air and repeatedly groaned "please, I can’t breathe."

Mr Floyd, whom police suspected of trying to pass a counterfeit bill to pay for cigarettes, was pronounced dead at a hospital shortly after the May 25 encounter.

Three fellow officers fired from the Minneapolis police department along with Mr Chauvin the next day were charged on Wednesday – each with aiding and abetting both second-degree murder and manslaughter.

Thomas Lane, J. Alexander Kueng and Tou Thao are also in custody. Aiding and abetting second-degree murder carries the same maximum punishment as the underlying offence – 40 years in prison.

Minnesota Attorney General Keith Ellison has requested bail of $US1 million ($A1.44 million) for each of the former officers.

"This is a significant step forward on the road to justice, and we are gratified that this important action was brought before George Floyd’s body was laid to rest," Benjamin Crump, lawyer for the Floyd family, said in a statement.

Mr Ellison told a news conference that winning a conviction "will be hard," noting that Hennepin County Attorney Mike Freeman, whose office filed the original charges against Mr Chauvin, is the only prosecutor in the state to have successfully convicted a police officer for murder.

Fully investigating the case "is going to take months", he said.

Mr Trump has said justice must be done in Mr Floyd’s case but also touted a hard line against violent protests, threatening to use the military to restore order.

But Defence Secretary Mark Esper said he did not back deploying troops to patrol the country.

"The option to use active duty forces in a law enforcement role should only be used as a matter of last resort and only in the most urgent and dire of situations. We are not in one of those situations now," he said.

-with agencies