US President Donald Trump been censored by social media platform Twitter after he “glorified violence” in a tweet, as riots in Minneapolis took an even more destructive turn on Friday (Australian time).

Trump labelled protestors rallying against the death of black man George Floyd in police custody as “thugs” and vowed, “when the looting starts, the shooting starts”.

But hours later, Twitter said the president’s tweet violated their rules.

“This Tweet violated the Twitter Rules about glorifying violence. However, Twitter has determined that it may be in the public’s interest for the Tweet to remain accessible,” a label blocking the tweet read.

The tweet by President Donald Trump censored by Twitter.

Earlier on Friday (Australia time), protesters invaded a police station and set it alight during the third day of rioting in the city, and neighbouring St Paul.

NBC reported that buildings on either side of the police station – the base of the four officers involved in the fatal arrest – were also burning.

Protesters massing outside the building briefly retreated under volleys of police tear gas and rubber bullets fired at them from the roof, only to reassemble and eventually attack the building head on, setting fire to the structure as police seemed to withdraw. Protesters were later observed on the roof.

Demonstrators had pushed down temporary fencing to gain entry to the precinct, while police fired tear gas from the ground and a rooftop.

“This demonstration has now turned into a confrontation with police here,” Minnesota Reformer journalist Ricardo Lopez told the ABC.

“Now police are pepper-spraying the crowd that had moved here from the downtown government centre, and it’s now become mayhem here in downtown Minneapolis.”

Protests and looting had already ravaged several blocks in the city, with scattered clashes elsewhere. The state’s governor called in the National Guard earlier on Thursday, to try to quell the rising violence.

A car and at least two other buildings in the vicinity were also set ablaze, and looters returned for a second night to a nearby Target discount store, left boarded up and vacant from the previous night, to make off with whatever remained inside.

US President Donald Trump has warned looters that he was prepared to send in the National Guard to quell the protests.

Tweet from @realDonaldTrump

Businesses across the twin cities of Minneapolis and St Paul have boarded up windows and doors to try to prevent looting.

Tweet from @CalebJHull

The anger about Mr Floyd’s arrest has also spread to other US cities, including New York, Los Angeles and Denver.

Earlier, Governor Tim Walz ordered National Guard troops to help police as local, state and federal law enforcement officials sought to ease racial tensions sparked by Monday night’s fatal arrest of 46-year-old Mr Floyd. The National Guard said it had sent 500 soldiers to the two cities.

Four city police officers involved in the incident, including one shown pressing his knee into Mr Floyd’s neck as he lay on the ground, were fired the next day.

An onlooker’s video of the arrest shows him Mr Floyd lying face down on the street, gasping for air and repeatedly groaning for help as he pleads, “Please, I can’t breathe.”

The officer pinned his neck to the ground for about eight minutes, until he grew still. Mr Floyd was pronounced dead at a hospital a short time later.

Minneapolis authorities have named the four officers involved in Mr Floyd’s arrest as Derek Chauvin, Thomas Lane, Tou Thao and J Alexander Kueng.

Local news media have identified Mr Chauvin as the officer who knelt on Mr Floyd’s neck. Mr Chauvin’s lawyer, Tom Kelly, declined comment in an email to Reuters.

Police department records posted online show 18 internal affairs complaints filed against Mr Chauvin, 16 of them closed without discipline.

A protester washes her eyes after being tear-gassed. Photo: AAP

Police Chief Medaria Arradondo has apologised to Mr Floyd’s family, conceding his department had contributed to a “deficit of hope” in Minnesota’s largest city, even before Mr Floyd’s deadly encounter with police.

“I am absolutely sorry for the pain, devastation and the trauma that Mr Floyd’s death has left on his family, his loved ones and our community,” he said.

Tweet from @Breaking911

Officials overseeing investigations from the US Justice Department, FBI, Minnesota Bureau of Criminal Apprehension and local prosecutors also appealed for calm at a joint news conference.

The investigation, which US Attorney General William Barr has designated a “top priority”, would focus on whether the arresting officers used the “colour of law” to deprive Mr Floyd of his civil rights, a federal crime, she said.

The announcement capped two days of unrest in which riot police firing tear gas and rubber bullets clashed with rock-throwing protesters who filled the streets in an outpouring of rage over Mr Floyd’s death.

Tweet from @CityMinneapolis

A second night of disturbances on Wednesday, punctuated by looting, vandalism and arson, began hours after Mayor Jacob Frey urged local prosecutors to file criminal charges in the case.

Protesters have pressed their demands that the four policemen be arrested and prosecuted.

“There is probable cause right now” to make those arrests, civil rights activist the Rev. Al Sharpton said as he addressed the crowd. “We’re not asking for a favour. We’re asking for what is right.”

Most protesters had been peaceful, while a core group have engaged in unruly behaviour, the police chief said.

United Nations High Commissioner for Human Rights Michelle Bachelet urged US authorities on Thursday to deal with “entrenched and pervasive racial discrimination” in America’s criminal justice system.

The Floyd case is reminiscent of the 2014 killing of Eric Garner, an unarmed black man in New York City who died after being put in a banned police chokehold as he, too, was heard to mutter, “I can’t breathe.”

Mr Garner’s dying words became a rallying cry for the Black Lives Matter movement that formed amid a wave of killings of African-Americans by police.

-with agencies