U.S. city of Minneapolis has entered the fourth day of unrest following the death of an unarmed black man George Floyd.

Floyd died on Monday after a white police officer arrested him.

An onlooker's cell phone recorded the incident showing the 46-year-old black man moaned, "Please, I can't breathe" and "Don't kill me" when the police officer was pressing his knee into Floyd's neck. However, the video didn't show how the confrontation started.

The protest began on Tuesday afternoon was peaceful at first, with demonstrators coming to the intersection where Floyd lost consciousness.

While thousands of protesters raged on Wednesday night, they burned fires and looted stores across the city and turned into a standoff outside the Third Precinct police station, about half a mile from where Floyd had been arrested.

Minnesota's governor activated the National Guard on Thursday to help restore order.

The demonstrations are now spreading across the U.S. Sympathy protests erupted on Wednesday in Los Angeles, and Thursday in Denver, with freeway traffic, blocked in both cities. In Phoenix, protesters faced off with police in riot gear at City Hall, and a rally was held at the Arizona state Capitol.

What's the reaction?

The death of Floyd spurred reactions from U.S. politicians and the UN.

White House press secretary Kayleigh McEnany told reporters on Thursday that President Donald Trump "was very upset" when he saw Floyd's death footage.

In the late-night of Thursday, President Trump tweeted he will "send in the National Guard" to restore order to Minneapolis. "When the looting starts, the shooting starts," Trump said, prompting Twitter to hide the message for, in its view, "glorifying violence."

He also accused Mayor Jacob Frey of lacking leadership, saying the mayor is very weak and radical. When talking to reporters, Frey responded, "Weakness is refusing to take responsibility for your own actions. Weakness is pointing your own finger at a time of crisis."

"Donald Trump knows nothing about the strength of Minneapolis. We are strong as hell," he added.

While Trump's main rival in the coming 2020 presidential election, Former Vice President Joe Biden compared the death of Floyd to Eric Garner, an unarmed black man who was placed in a police chokehold in New York in 2014.

Biden said the death of Floyd showed the black lives in the U.S. are under threat every day.

When talking to reporters in Capital, Speaker Nancy Pelosi said Floyd was murdered. "We saw it on TV — him being murdered on TV."

However, a short time later, she told CNN in the interview that Floyd's death is "an execution."

Calling the incident as the latest in "a long line of killings of unarmed African Americans by U.S. police officers and members of the public," UN High Commissioner for Human Rights Michelle Bachelet has also condemned Floyd's death.

She urged the U.S. authorities to take serious action to stop such killings and to ensure justice is done when they do occur.