George Floyd death: US cities erupt in more violence as Trump decries 'lowlifes'

President Donald Trump stepped up the pressure on state governors to crack down after a week of unrest set off by the death of George Floyd, demanding New York call up the National Guard to stop the "lowlifes and losers".

As cities around the US witnessed a seventh straight night of peaceful demonstrations and bursts of theft, vandalism and attacks on police, the president amplified his hardline calls of a day earlier, in which he threatened to send in the military to restore order if governors didn't do it.

"NYC, CALL UP THE NATIONAL GUARD," he tweeted. "The lowlifes and losers are ripping you apart. Act fast! Don't make the same horrible and deadly mistake you made with the Nursing Homes!!!"

In New York on Monday night (US time), people smashed shop windows near the Rockefeller Centre and breached the doors of Macy's flagship store on 34th Street despite the first curfew in the city in decades. Police said nearly 700 people were arrested and several officers injured in the overnight violence.

New York and other cities braced for more unrest after nightfall Tuesday and Mayor Bill de Blasio extended an 8pm curfew all week.

"We're going to have a tough few days," he warned, but added: "We're going to beat it back." He said he would do that with the help of community leaders, urging them to step forward: "Create peace. Stand up."

More than 20,000 National Guard members have been called up in 29 states to deal with the unrest. New York is not among them. De Blasio has said he does not want the National Guard, and Govenor Andrew Cuomo has said he will not send it into the city against the mayor's wishes.

During the violence that gripped communities from coast to coast again on Monday night, police officers were shot, run over and showered with rocks and bottles.

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In Philadelphia, hundreds of protesters spilled onto a highway in the heart of the city. In Atlanta, police fired tear gas at demonstrators. In Nashville, more than 60 National Guard members put down their riot shields at the request of peaceful protesters. And at a demonstration in Buffalo, New York, an SUV plowed into a group of officers, injuring three.

"We have been sitting on a powder keg for some time and it has burst," Philadelphia Police Commissioner Danielle Outlaw said.

Some protesters framed the burgeoning movement as a necessity after a string of killings by police.

"I fear for my safety every time I get in the car to go for a drive. I fear of getting pulled over. I fear for all 10 of my brothers' and sisters' lives, for my parents' lives!" 19-year-old Amari Burroughs of Parkland, Florida, said Tuesday as she prepared for another protest. "My goal is to use my voice and my leadership to make this world safer so that one day I can bring children here and won't have to fear for their safety."

Democratic governors and mayors rejected Trump's threat to send in the military, questioning whether the federal government has such authority and warning that such a step would be incendiary.

De Blasio said military troops would create "a dangerous scenario", while Colorado governor Jared Polis and Denver Mayor Michael Hancock warned such a move would just lead to more violence and destruction.

"Denver is not Little Rock in 1957, and Donald Trump is not President Eisenhower. This is a time for healing, for bringing people together, and the best way to protect civil rights is to move away from escalating violence," they said.

A senior White House official said Tuesday that despite Trump's threats, the goal was to pressure governors to deploy more National Guard members. The president was not rushing to use the Insurrection Act to send in the military, according to the official, who spoke on condition of anonymity.

The unrest in Minneapolis, meanwhile, appeared to stabilise on the same day Floyd's brother made an impassioned plea for peace at the spot where a white police officer, Derek Chauvin, put his knee on the handcuffed black man's neck for several minutes, even after he stopped moving and pleading for air.

Chauvin has been charged with murder. Minnesota Attorney General Keith Ellison said Tuesday that prosecutors were working as fast as they can to determine if the three other officers at the scene should be charged, too. All four have been fired.

The death toll from the unrest rose to at least nine, including two people killed in a Chicago suburb. More than 5600 people nationwide have been arrested over the past week for such offences as stealing, blocking highways and breaking curfew, according to a count by the Associated Press.

An officer was shot shortly before midnight near the Circus Circus casino in Las Vegas. Police had no immediate word on the officer's condition. Four officers were shot in St Louis but were expected to recover.

Cities struggled to keep police in line and avoid instances of excessive force.

The police chief in Louisville, Kentucky, was fired after a restaurant owner was killed by police and National Guard members enforcing a curfew. In Richmond, Virginia, the police chief said officers who used tear gas on a group of peaceful protesters would be disciplined. In Atlanta, six officers were charged Tuesday after a video showed authorities dragging two young people from a car during protests.

Sending the military into the states would mark a stunning federal intervention rarely seen in modern American history. It drew comparisons to 1968, when Richard Nixon ran as the law-and-order candidate in the aftermath of riots following the assassination of the Reverend Martin Luther King jnr.

Federal law allows presidents to send the military into states to suppress an insurrection or if a state is defying federal law, legal experts said. But some state officials asserted that the president does not have a unilateral right to send in troops against the will of local governments.

- AP