The Minneapolis police officer who knelt on the neck of George Floyd for nearly nine minutes will now be charged with second-degree murder, while his three colleagues will also face charges, court documents show.

The death of Mr Floyd has ignited protests across the United States over systemic racism and police brutality.

"Minnesota Attorney General Keith Ellison is increasing charges against Derek Chauvin to 2nd degree in George Floyd's murder and also charging other 3 officers," US Senator Amy Klobuchar said

"This is another important step for justice."

Mr Chauvin was charged last week with third-degree murder, which is roughly akin to manslaughter.

A charge of second-degree murder does not suggest premeditation but carries stiffer penalties.

Court documents show the second-degree murder charge was added to the prior charges.

The three other officers have been charged with aiding and abetting second-degree murder, documents show.

In a statement, Mr Floyd's family described news of the new charges as a "bittersweet moment."

"This is a significant step forward on the road to justice, and we are gratified that this important action came before George Floyd's body was laid to rest," the statement said.

The statement, issued by family attorney Ben Crump, also said that Mr Ellison would consider elevating the charge to premeditated murder "if the evidence supports it."

The family urged protesters to "raise their voices for change in peaceful ways."

Tens of thousands of demonstrators defied night-time curfews yesterday in several US cities.

But the demonstrations were largely peaceful, and while there were tense standoffs with law enforcement, the protests did not feature the looting or clashes with police of previous days.

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Tens of thousands of people took to the streets of cities for an eighth consecutive night of protests, with large marches and rallies held in Los Angeles, Philadelphia, Atlanta and Seattle.

In Washington, DC, protests were held near the park where demonstrators were cleared out by police on Monday to make a path for President Donald Trump so he could walk from the White House to a historic church for a photo.

Although rallies on behalf of Mr Floyd and other victims of police brutality have been largely peaceful during the day, after dark each night crowds have turned to rioting, vandalism, arson and looting.

On Monday night, five police officers were hit by gunfire in two cities.

Outside the US Capitol building yesterday afternoon a throng took to one knee, chanting "silence is violence" and "no justice, no peace," as officers faced them just before the government-imposed curfew.

The crowd remained in Lafayette park and elsewhere in the capitol after dark, despite the curfew and vows by President Trump to crack down on what he has called lawlessness by "hoodlums" and "thugs", using National Guard or even the US military if necessary.