Flames ripped through a Minnesota police station and seven protesters were shot in Kentucky as unrest spread across the United States over the deaths of black people during police encounters.

Officers abandoned the building in the city of Minneapolis late on Thursday before demonstrators barged through barriers, breaking windows and chanting slogans. A fire broke out, which soon became an inferno that engulfed the structure.

A Minneapolis police officer holds his knee to the neck of George Floyd, who died in police custody.
A Minneapolis police officer holds his knee to the neck of George Floyd, who died in police custody. Facebook/Darnella Frazier / Darnella Frazier

The protests entered their fourth day on Friday and have spread beyond Minnesota, with protests breaking out in several states across the country, including Denver, Colorado and Phoenix.

In Kentucky, seven people were hit by gunfire at a protest on Thursday over the death of Breonna Taylor -- a black woman who was shot after police entered her home in March, local media reported.

An injured protester is helped by others as demonstrators clash with police during rallies on May 27, 2020 against the death of George Floyd in Minneapolis
An injured protester is helped by others as demonstrators clash with police during rallies on May 27, 2020 against the death of George Floyd in Minneapolis AFP / Kerem Yucel

One of those wounded was in a critical condition, according to the Louisville Metro Police Department. It is not yet clear who fired the shots.

Police responded with a Twitter post asking the city to "please choose peace," alongside a video message from a family member of the woman killed. She asked those in the streets to "go home and be safe and be ready to keep fighting."

A rubber bullet is seen on the ground during a demonstration in Minneapolis on May 27, 2020 calling for justice for George Floyd
A rubber bullet is seen on the ground during a demonstration in Minneapolis on May 27, 2020 calling for justice for George Floyd AFP / Kerem Yucel
A police officer holds a projectile launcher during a demonstration in Minneapolis on May 27, 2020 calling for justice for George Floyd following his death
A police officer holds a projectile launcher during a demonstration in Minneapolis on May 27, 2020 calling for justice for George Floyd following his death AFP / kerem yucel

Thousands joined the protests in Minnesota, which were triggered by the Monday death of 46-year-old George Floyd after being arrested on suspicion of using a counterfeit banknote.

A video taken by a bystander shows an officer kneeling on his neck as he is pinned to the ground. At one point Floyd is heard saying he cannot breathe.

Protestors cry from from tear gas during a demonstration in Minneapolis on May 27, 2020 calling for justice for George Floyd
Protestors cry from from tear gas during a demonstration in Minneapolis on May 27, 2020 calling for justice for George Floyd AFP / Kerem Yucel

As unrest spread, President Trump tweeted: "These THUGS are dishonouring the memory of George Floyd and I won't let that happen," in apparent reference to protesters in Minnesota, adding the state's governor has the backing of the military.

"Any difficulty and we will assume control but, when the looting starts, the shooting starts."

Two police officers stand on the roof of the Third Police Precinct holding a projectile launcher during a demonstration against the death of George Floyd, in Minneapolis, Minnesota on May 27, 2020
Two police officers stand on the roof of the Third Police Precinct holding a projectile launcher during a demonstration against the death of George Floyd, in Minneapolis, Minnesota on May 27, 2020 AFP / Kerem Yucel

Twitter flagged the tweet for violating its rules on glorifying violence a few hours after it appeared.

Minnesota's governor Tim Walz earlier called up 500 of the state's National Guard, but after Trump tweeted he defended his decision not to put them on the streets as fire gripped the police station.

"Bricks and mortar are not as important as life," Minneapolis mayor Jacob Frey said at a press conference, adding that officers had been deployed in the city to prevent looting.

US President Donald Trump says the death of George Floyd, a black man who died while an officer kneeled on his neck, is "a very, very sad event". Trump made the comments while touring the Kennedy Space Center before SpaceX's planned launch to the Internat
US President Donald Trump says the death of George Floyd, a black man who died while an officer kneeled on his neck, is "a very, very sad event". Trump made the comments while touring the Kennedy Space Center before SpaceX's planned launch to the International Space Station, which was eventually scrubbed due to fears of a lightning strike. DC POOL /

He added that the "anger and frustration" on the streets must be understood, but that the looting was unacceptable.

"Our communities cannot and will not tolerate it. These are businesses, these are community institutions that we need," Frey said.

The City of Minneapolis called for people in the third precinct -- where the police building is located -- to leave for their own safety, saying "explosive materials" could be inside and gas lines to the area had been cut.

Outbreaks of violence have gripped the city as tensions rose since Floyd's killing.

On Wednesday, demonstrators clashed with law enforcement, looted stores and set fire to shops and a construction site. They 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.

The Justice Department on Thursday promised to carry out a "robust" investigation into Floyd's death, saying they will make the case a top priority.

Democrats have also called for a probe into the deaths of two other black people -- Ahmaud Arbery, who was shot by two white men in the city of Brunswick in Georgia, and Breonna Taylor.

Floyd's family has demanded the officers present when he died face murder charges. All of them have been fired.

"I have not slept in four days, and those officers, they're at home sleeping," he said.

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