Protests erupted across the United States on Thursday night as anger over the death of George Floyd, an unarmed Black man, intensified, with some demonstrators gaining access to a police precinct in Minneapolis, Minnesota, and setting sections of the building on fire.

Floyd died on Monday after a white Minneapolis police officer used his knee to pin Floyd's neck to the ground for several minutes.

A video of the incident shows Floyd pleading with officers, saying "I can't breathe" before going motionless with the officer's knee still on his neck.

The four officers involved were swiftly fired, but Floyd's family, community leaders and residents are calling for arrests to be made.

"These officers, they need to be arrested right now, the people want justice right now," Philonese Floyd, George's brother, told CNN on Thursday morning.

"They need to be convicted and get the death penalty," Philonese said.

Hundreds of protesters marched in downtown Minneapolis for a third night on Thursday, demanding justice and an end to police violence.

"Say his name. George Floyd," protesters chanted. "I can't breathe."

Video shared on social media showed protesters stopping their march at one point, kneeling and raising a fist in a moment of silence.

As the sun set, a large fire could be seen near the third precinct of the Minneapolis Police Department (MPD). A live video stream by Unicorn Riot, an independent media organisation, showed protesters eventually entering the precinct and alarms and sprinklers going off as some rooms were set ablaze. Police could not be seen in the building, and the MPD did not immediately comment on reports that police had retreated.

The City of Minneapolis urged protesters to retreat from the area over unconfirmed reports that gas lines had been cut.

"We're hearing unconfirmed reports that gas lines to the Third Precinct have been cut and other explosive materials are in the building," the city tweeted. "If you are near the building, for your safety, PLEASE RETREAT in the event the building explodes."

Protesters outside the police building could be heard yelling: "We're sick and tired of being sick and tired."

Minnesota Governor Tim Walz called in the US National Guard on Thursday to support local authorities as the protests escalated.

Earlier on Thursday, a small group of protesters "occupied" the space outside the home of Hennepin County Attorney Mike Freeman, who will handle the case, to demand criminal charges for the four officers - identified as Derek Chauvin, Thomas Lane, Tou Thao and J Alexander Kueng - involved.

"We aren't going anywhere until Mike Freeman prosecutes and charges the officers," protesters said in a Facebook Live video, with at least one tent put up on the sidewalk outside the county attorney's home.

Freeman's office said in a statement on Tuesday it was "shocked and saddened by what appeared in a recent video".

It said it would make a decision on prosecution after it receives the completed findings of the investigations by the Minnesota Bureau of Criminal Apprehension and the FBI.

Freeman said on Thursday his office would work through the case "as expeditiously, as thoroughly as justice demands".

"We just can't rush this," Freeman said. "These need to be done right. Please give me and give me the United States attorney time to do this right and we will bring you justice."

Police response to protests

Thursday's protest followed similar demonstrations in Minneapolis that have taken place for the last two days.

Floyd's death has been compared with that of Eric Garner, an unarmed Black man who died in 2014 after police put him in a chokehold. Some of Garner's last words were: "I can't breathe."

While the protests have started peaceful, they have descended into chaos with reports of looting, arson and vandalism.

Police have used tear gas and non-lethal projectiles to disperse the crowds, drawing anger from local residents.

"The way the police - MPD - have been treating people, having been treating the community, have been treating young people is incredibly frustrating," said Ramla Bile, a Somali American Minneapolis resident.

"Seeing all the sophisticated gear that they have is disgusting," she told Al Jazeera.

Bile said she found it especially frustrating that "for a city where we are continuing to talk about inequity and disparities ... we continue to funnel money to the police" instead of investing in these communities.

Protests also took place on Thursday in cities across the US, including Columbus, Ohio, Oakland, California, and New York City, where dozens of people were reportedly arrested.

Hundreds also rallied in Louisville, Kentucky, to protest against police brutality and draw attention to the killing of Breonna Taylor who was shot dead by police in March as they served a search warrant. At least seven people were shot at the protest, police said on Thursday. One person is in critical condition and no arrests have been made, police said.

In Denver, Colorado, hundreds of people descended on the state capitol. Police confirmed that shots were fired near the area where the protest was taking place. It is unclear if the shots were related to the protest. Police said there were no immediate reports of injuries.

Several more protests are scheduled for Friday and over the weekend.