Hundreds of mourners in Minneapolis have remembered George Floyd, the black man whose death in police custody set off a wave of nationwide protests that reached the doors of the White House and ignited a debate about race and justice.

"Everyone wants justice, we want justice for George, he's going to get it," Philonise Floyd, one of Mr Floyd's brothers, told a memorial service at a chapel in the Minnesota city's North Central University.

"It’s crazy man, all these people came to see my brother, it’s amazing he touched so many hearts," said the brother, wearing a badge with a photo of his brother and the words "I can't breathe" on his lapel.

Mr Floyd's death in May has become the latest flashpoint for rage over police brutality against African-Americans, propelling the issue of race to the top of the political agenda five months before the US presidential election on 3 November.

Derek Chauvin, 44, was fired from the Minneapolis police force and charged with second-degree murder after being filmed in a widely circulated video kneeling on Mr Floyd's neck for nearly nine minutes as Mr Floyd gasped for air and repeatedly groaned,"Please, I can't breathe".

Police say they suspected Mr Floyd, 46, of using a counterfeit bill to pay for cigarettes.

Huge crowds have defied curfews and taken to the streets of cities across the country for nine nights in sometimes violent protests that prompted President Donald Trump to threaten to send in the military.

"It's going to take a united effort inside the courtroom and outside the courtroom to get justice for George Floyd," Ben Crump, a lawyer for Mr Floyd's family, told the memorial service.

Prosecutors yesterday levelled new charges against four former Minneapolis police officers implicated in the killing.

Today, the three officers charged with aiding and abetting the murder of Mr Floyd made their first appearances in court, where bail was set at $1 million but would be lowered to $750,000 if they agreed to certain conditions, including forfeiting any personal firearms.

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Reverend Al Sharpton (L) arrives with family members of George Floyd for the memorial service
A painting of George Floyd at a makeshift memorial outside the store where he died

In New York City, which has been hit by looting during the protests, thousands of people attended a memorial event in a Brooklyn park for Mr Floyd.

Many knelt in the grass in the afternoon sunshine in a symbol of protest against police behaviour and chanted, "No Justice. No Peace."

Mayor Bill de Blasio, who some in the crowd had booed when he arrived on stage, called on white people to do more to understand African-American communities.

"For all of us who have not walked a mile in the shoes of the black community, or communities of colour, all of us who know white privilege, we need to do more, because we don't even fully recognise the daily pain that the racism in this society causes," he said.

US Attorney General William Barr said today that foreign interests and "extremist agitators" were taking over the protests.

Mr Barr, the country's top law enforcement official, said foreign interests and "extremist agitators" affiliated with movements like Antifa, which opposes authoritarianism, have sought to widen divisions in US society during the protests.

"We have seen evidence that Antifa and other similar extremist groups as well as actors of a variety of different political persuasions have been involving in instigating and participating in violent activity," Mr Barr said without offering details.

In another racially charged case that has gained national attention, a court heard today that one of the white men charged in the murder of unarmed black man Ahmaud Arbery in Georgia used a racial slur after shooting the man and before police arrived at the scene.

Special Agent Richard Dial, an investigator for the prosecution, quoted William Bryan as saying fellow defendant Travis McMichael uttered the slur after shooting Mr Arbery in February.

Services for Mr Floyd are expected to stretch across six days and three states.

Memorials will also be held on Saturday in Hoke County, North Carolina, where Mr Floyd's sister lives, and in Houston on Monday, near where Mr Floyd had lived.

A funeral is planned for Tuesday with private services at an undisclosed location.