Tens of thousands of protesters have taken to the streets of US cities for an eighth consecutive night over the death of a black man in police custody, defying curfews and threats of a military crackdown.

There were long marches and rallies in New York, Washington, Los Angeles, Philadelphia, Atlanta and Seattle, invoking the name of George Floyd, and other police victims.

On Tuesday night, twice as many people turned out in Washington as on Monday, when police cleared demonstrators from a park with tear gas and rubber bullets to make a path for President Donald Trump so he could walk from the White House to a nearby church for a photo.

The defiant crowd in Lafayette Park and elsewhere in the capitol ignored threats by Mr Trump to use the military to crack down on what he has called lawlessness by “hoodlums” and “thugs”.

Tweet from @JennLeeTV

Outside the US Capitol building on Tuesday, a throng took to one knee, chanting “silence is violence” and “no justice, no peace,” as officers faced them.

In New York, police held thousands of marchers on the Manhattan Bridge in a stand-off as NYPD helicopters whirred overhead. Demonstrators trying to take the bridge to Manhattan Island after the city’s 8pm curfew were halted by police barricades at both sides, according to reports.

They were eventually allowed to leave about 11pm.

Across the country, there were reports of tear gas being used on demonstrators in Portland after some of the marchers threw bottles and bats at police. There were also reports 5000 people had occupied a local bridge.

Tweet from @realDonaldTrump

On Hollywood Boulevard in Los Angeles, hundreds of people filled the street from kerb to kerb. Others gathered outside Los Angeles Police Department headquarters, some of them hugging and shaking hands with a line of officers outside.

Elsewhere, National Guard troops patrolled darkened LA streets.

Tuesday night’s protests came as the Pentagon said it had moved about 1600 US Army troops into the Washington, DC, region – a day after Mr Trump said he would bring in the military to end the “domestic terrorism”.

Tweet from @nikkifirewall

Mr Trump also urged state governors to call out the National Guard to quell the violence. Many governors openly defied his call but the head of the US National Guard said on Tuesday (local time) that 18,000 Guard members were assisting local law enforcement in 29 states.

Mr Trump’s militaristic rhetoric and the growing role of the US armed forces has alarmed some current and former officials.

“America is not a battleground. Our fellow citizens are not the enemy,” Martin Dempsey, a retired four-star general and former chairman of the joint chiefs of staff, wrote on Twitter.

But Mr Trump has appeared to ignore such calls, going on a Twitter spree late on Tuesday to blame the “radical left”, “hoodlums” and “low-lifes and losers” for the turmoil. He also thanked himself for resolving the tensions in Washington.

Tweet from @realDonaldTrump

Pleas for calm

Some of those gathered at the site of Mr Floyd’s killing in Minneapolis invoked the non-violent message of the late US civil rights leader Martin Luther King Jr., who was assassinated in 1968, as the only way forward.

“He would be truly appalled by the violence because he gave his life for this stuff,” said Al Clark, 62, a black man who drove to the memorial with one of King’s speeches blaring from his truck.

“But I can understand the frustration and anger.”

On Tuesday, an Reuters/Ipsos poll found a majority of Americans sympathise with the protests.

The survey, conducted on Monday and Tuesday, found 64 per cent of American adults were “sympathetic to people who are out protesting right now”, while 27 per cent said they were not and 9 per cent were unsure.

More than 55 per cent of Americans said they disapproved of Mr Trump’s handling of the protests, while just a third approved.

In Minneapolis, Roxie Washington, the mother of Mr Floyd’s six-year-old daughter, Gianna, told a news conference he was a good man.

“I want everybody to know that this is what those officers took from me,” she said, sobbing. “Gianna does not have a father. He will never see her grow up, graduate.”

Mr Floyd died last Monday after a white policeman pinned his neck under a knee for nearly nine minutes in Minneapolis on May 25.

The officer who knelt on Mr Floyd, 44-year-old Derek Chauvin, has been charged with third-degree murder and second-degree manslaughter. Three other officers involved were fired but not yet charged.

Mr Trump walks from the White House to St John’s Church on Monday (US time).

A former president’s support

Former Republican president George W. Bush said Mr Floyd’s death  reflected a “shocking failure” in the US – and called for the protesters to be heard.

Without mentioning Mr Trump by name, Mr Bush said it was out of step with the country’s values to have driven protesters from Lafayette Square, across from the White House, on Monday just before Mr Trump walked through for a photo opportunity.

“The only way to see ourselves in a true light is to listen to the voices of so many who are hurting and grieving,” Mr Bush said in a statement on Tuesday.

“Those who set out to silence those voices do not understand the meaning of America – or how it becomes a better place.”

On Tuesday, The Washington Post reported that the order to remove the protesters came from Attorney General William Barr.

Mr Bush said he and his wife, Laura, were anguished at Mr Floyd’s “brutal suffocation”.

“It is time for America to examine our tragic failures – and as we do, we will also see some of our redeeming strengths,” said the 43rd US president, who served from 2001 to 2009.

“This tragedy – in a long series of similar tragedies – raises a long overdue question: How do we end systemic racism in our society?”

-with AAP