United States President Donald Trump on Friday warned that he would "send in the National Guard and get the job done right" if protests in Minnesota were not curbed.

He was referring to ongoing demonstrations against the killing of George Floyd, a 46-year-old black man who died after Minneapolis police arrested him on Monday on suspicion of using a counterfeit banknote. Police handcuffed him and held him to the ground, with a bystander video showing an officer pressing his knee on Floyd's neck.

The videos showed Floyd saying that he couldn't breathe until he went silent and limp. He was later declared dead.

In a two-part tweet, Trump blamed Minneapolis "very weak Radical Left" mayor, Jacob Frey, for failing to control the protests that had grown violent in some parts of the city. The US president further said that he had told Minnesota's Governor Tim Walz that the "military is with him all the way" and went on to warn that law enforcement will resort to shooting if violent protests did not stop.

"These THUGS are dishonoring the memory of George Floyd, and I won’t let that happen," Trump tweeted. "Just spoke to Governor Tim Walz and told him that the Military is with him all the way. Any difficulty and we will assume control but, when the looting starts, the shooting starts. Thank you!"

President Trump's second tweet was flagged by Twitter for violating the platform's rules regarding "glorifying violence". It was not taken down, however.

A police precinct in Minnesota went up in flames late on Thursday in the third day of demonstrations as the so-called Twin Cities of Minneapolis and St. Paul seethed over the shocking police killing of a handcuffed black man.

The precinct, which police had abandoned, burned after a group of protesters pushed through barriers around the building, breaking windows and chanting slogans. A much larger crowd demonstrated as the building went up in flames.

The crowd was protesting the death of George Floyd, a 46-year-old black man who died after Minneapolis police arrested him on Monday on suspicion of using a counterfeit banknote. Police handcuffed him and held him to the ground, with a bystander video showing a officer pressing his knee on Floyd's neck.

The videos showed Floyd saying that he couldn't breathe until he went silent and limp. He was later declared dead.

Hundreds of people had begun marching in Minneapolis in the late afternoon — many wear masks as protection against the novel coronavirus — while in St. Paul, just to the east, police said there was ongoing looting as multiple fires were reported.

But later in the evening, a large crowd demonstrated outside the city's Third Precinct.

“Shortly after 10:00 pm tonight, in the interest of the safety of our personnel, the Minneapolis Police Department evacuated the 3rd Precinct of its staff,” city police said in a statement.

Probe under way

Officials assured angry residents that investigations into Floyd's death were underway, and warned that violence would not be tolerated.

“We know there's a lot of anger. We know there's a lot of hurt,” said St. Paul Police Chief Todd Axtel.

“But we can't tolerate people using this as an opportunity to commit crimes,” he said.

At the request of both cities, Minnesota Governor Tim Walz called up hundreds of National Guard troops and state police to help with security.

“George Floyd's death should lead to justice and systemic change, not more death and destruction,” Walz said.

Outrage spreads

Floyd's family demanded the officer and three others who were present, all since fired from their jobs, face murder charges.

“You know, I want an arrest for all four of those officers tonight. A murder conviction for all four of those officers. I want the death penalty,” Floyd's brother, Philonise Floyd, told CNN.

“I have not slept in four days, and those officers, they're at home sleeping,” he said. “I can't stand for that.”

“But people are torn and hurting because they are tired of seeing black men die, constantly, over and over again.”

Two African American leaders of national stature, Jesse Jackson and Al Sharpton, arrived in Minneapolis and urged more protests.

“We told the governor you must call murder a murder,” Jackson told an audience at the Greater Friendship Missionary Baptist Church.

“When you put... your foot down somebody's neck until they can't breathe no longer, you murdered them,” he said.

Sharpton said videos were all the evidence needed to arrest the police officers involved.

“We are going to make sure that this prosecution goes down,” said Sharpton.

Case is 'top priority'

Local and federal investigators said they were working the explosive case as fast as they could.

“The Department of Justice has made the investigation in this case a top priority,” said Erica MacDonald, the US federal attorney for Minnesota.

“To be clear, President (Donald) Trump, as well as Attorney General William Barr, are directly and actively monitoring the investigation in this case.”

The White House said Trump was “very upset” upon seeing the “egregious, appalling” video footage and demanded his staff see that the investigation was given top priority.

“He wants justice to be served,” Trump's press secretary Kayleigh McEnany told reporters.

Tear gas and rubber bullets

Demonstrators clashed with law enforcement, looted stores and set fires to shops and a construction site overnight on Wednesday in the busy Lake Street corridor of Minneapolis, and were met with police tear gas and rubber bullets.

One person died of a gunshot wound, and police were reportedly investigating whether he was shot by a store owner.

Some stores, including Minneapolis-based Target, afterward announced they would close multiple locations, as the US Postal Service suspended service to some areas and bus services were discontinued through the weekend in parts of the city.

Floyd's killing evoked memories of riots in Ferguson, Missouri in 2014 after a policeman shot dead an African American man suspected of robbery, and the case earlier the same year of New Yorker Eric Garner, who died after New York police put him in an illegal chokehold as they tried to detain him for selling cigarettes.

Sympathy protests erupted in other cities.

Several hundred people demonstrated in New York's Union Square on Thursday, leading to at least five arrests.

In Los Angeles, where there are longstanding tensions between law enforcement and black residents, protesters marched on Wednesday on downtown and briefly blocked a major freeway.

Activists were planning a rally on Friday in downtown Washington near the White House.

And protesters gathered in Denver, Colorado and Phoenix, Arizona Thursday evening, according to CNN.

Ilhan Omar, a black Somalia native who represents Minneapolis in Congress, called for calm but said there was “extreme frustration” in the community over the incident.

“Anger really is boiling over because justice still seems out of reach,” she said.