US president Donald Trump has threatened to take control of Minneapolis as protesters take over the city in the wake of the killing of George Floyd.

Tensions exploded on a third day in Minneapolis, sparked by the death of Floyd, an African-American man who was pinned by a white police officer to the ground before his death.

City officials warned that a police station which was stormed by rioters and set alight could now explode as protests over Floyd's death spread across America.

President Trump waded in on the escalating violence in Minneapolis as he warned he would step in and take over if officials did not bring the rioting under control.

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He blasted the "Radical Left Mayor" Jacob Frey saying he needs to "get his act together" while slamming protesters for "dishonoring the memory" of Floyd and warning "when the looting starts, the shooting starts".

"I can't stand back & watch this happen to a great American City, Minneapolis. A total lack of leadership. Either the very weak Radical Left Mayor, Jacob Frey, get his act together and bring the City under control, or I will send in the National Guard & get the job done right.....," the president tweeted.

"These THUGS are dishonoring the memory of George Floyd, and I won't let that happen. 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!"

Video uploaded to social media shows protesters taking control of the 3rd Precinct of the Minneapolis Police Department and setting it alight.

The four officers involved in the death of George Floyd earlier this week worked at the 3rd Precinct.

Video from outside the police station show protesters chanting "I can't breathe", in reference to Floyd's final words before he died.

Breaking 911 reported Minneapolis police and firefighters have been told to stand down as rioters took over areas of the city and video shows the National Guard entering the city.

Police officer's actions have led to widespread protests in the US as demonstrators demand justice for George Floyd. Video / AP

Earlier, incredible footage was broadcast live on the Today Show. It showed a black man laying on a hill after being stabbed by a white man.

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Police had been avoiding the area in downtown Minneapolis, leaving the victim bleeding on the ground for 15 minutes before help arrived.

The vision showed a shirtless man being arrested, but even as police attempted to intervene they were pelted with rocks and bricks.

Mr Floyd's has caused outrage across the US and protests have sprung up in other cities, with hundreds of demonstrators calling for action against the US police.

In Los Angeles on Thursday protesters were filmed burning the American flag at a rally in Downtown LA, as more protests also kicked off in Memphis in Tennessee.

A large fire burns in Minneapolis during a third night of unrest. Photo / AP
A large fire burns in Minneapolis during a third night of unrest. Photo / AP

The demonstrations began after footage emerged on Tuesday showing Mr Floyd being pinned to the ground by a white police officer who was kneeling on his neck.

Mr Floyd, 46, had begged to be let go during the incident, telling the officer, "I can't breathe", before he lost consciousness. Terrified bystanders watched on, telling the officer he was bleeding from the nose and struggling. The arrest lasted for almost nine minutes.

He was then put on a stretcher and taken to the Hennepin County Medical Centre where was pronounced dead.

A reporter from the Nine network, Tim Arvier, was in the thick of riots as the stabbing unfolded today.

He was caught between police and protesters who threw large objects and had to be physically pushed back.

Earlier, he described the scene outside a Target that had been looted as "absolute lawlessness".

A man jumps off a burning car in a Target parking lot in Minneapolis during a third night of unrest following the death of George Floyd. Photo / AP
A man jumps off a burning car in a Target parking lot in Minneapolis during a third night of unrest following the death of George Floyd. Photo / AP

Arvier said looters had been walking into the department store and filling up shopping trolleys amid the chaos.

He said many of the looters had recently lost their jobs due to the coronavirus pandemic and were using the opportunity to get food.

Outside the store, cars were on fire and police were nowhere to be seen.

Rioting had also reportedly broken out in the Minnesota city of St Paul on Thursday afternoon, where police fired tear gas on demonstrators.

Smoke rises from fires as seen from University Avenue in St. Paul, Minn. Photo / AP
Smoke rises from fires as seen from University Avenue in St. Paul, Minn. Photo / AP

Police used batons and barricaded a department store in the city and were using squad cars to block entrances.

In video shared on social media, officers sprayed pepper spray from the window of a squad car at large crowds.

After days of continued unrest in the city, the Governor of the state of Minnesota today signed an executive order activating the National Guard to move into Minneapolis.

The National Guard will be providing "personnel, equipment, and facilities needed to respond to and recover" to the civil upheaval caused by the protests.

At a morning news briefing, Police Chief Medaria Arradondo said he was "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".

Leaders in the city have been urging the public for calm in recent days, according to the report.

The lawyer for Mr Floyd's family, Ben Crump, urged protesters not to engage in violence.

"We don't need that. We need people focused on getting justice," he said on Thursday.

He's also calling for an independent investigation and says he doesn't trust the city of Minneapolis.

PROTESTERS BURN FLAG IN LOS ANGELES

In Los Angeles protesters gathered and burned an American flag, in a video that has now gone viral on Twitter.

The flag burning demonstration was part of a larger Black Lives Matter protest that took place in the Downtown area of Los Angeles, where protesters blocked off the 101 Freeway and climbed onto California Highway Patrol cars.

One Highway Patrol car had its windows smashed, according to a report from the Hill as protesters climbed onto the roof of the vehicle.

Hundreds of protesters were involved in the Los Angeles demonstrations according to the report.

At a Black Lives Matter protest in Memphis on Thursday night protesters marched to the city's Midtown Police Station chanting, "I can't breathe".