Thousands of demonstrators gathered on Wednesday for a second night of protests over the killing in the US city of Minneapolis of a handcuffed black man by a policeman who held him to the ground with a knee on his neck.

Earlier in the evening, a peaceful protest descended into disarray and looting, after an Auto Zone had caught fire.

They pushed protesters back as the crowd grew, a day after police fired rubber bullets and tear gas on thousands of demonstrators angered by the latest death of an African-American at the hands of US law enforcement.

The demonstration followed the death of 46-year-old George Floyd on Monday, which came after a white officer pinned the handcuffed father of two to the pavement outside of a market where employees had called police about a counterfeit bill.

The police encounter was caught on a viral video that has sparked national outrage and inflamed existing tensions in a community where cops have long been accused of racism.

Taking to Twitter, President Donald Trump said Floyd’s death “sad and tragic”, as outrage spread across the country over a bystander’s cellphone video of his killing on Monday while in the custody of four white police officers.

All four have been fired, as prosecutors said they had called in the FBI to help investigate the case, which could involve a federal felony civil rights violation.

“I would like those officers to be charged with murder, because that’s exactly what they did,” Bridgett Floyd, his sister, said on NBC television.

“They murdered my brother…. They should be in jail for murder”, she further added

The case was seen as the latest example of police brutality against African Americans, which gave rise six years ago to the Black Lives Matter movement.

Floyd had been detained on a minor charge of allegedly using a counterfeit $20 bill to make a purchase at a convenience store.

In the video, one of the policemen hold him to the ground while another presses his knee to Floyd’s neck.

“Your knee in my neck. I can’t breathe…. Mama. Mama,” Floyd pleaded.

Democratic presidential candidate Joe Biden said the FBI needs to thoroughly investigate the case.

Biden said, “It’s a tragic reminder that this was not an isolated incident, but part of an engrained systemic cycle of injustice that still exists in this country”.

“We have to ensure that the Floyd family receive the justice they are entitled to”, the former Vice President further added.

Democratic Senator Kamala Harris also condemned the incident and called the policeman’s using his knee on Floyd’s neck “torture.”

“He was begging to be able to breathe,” she said.

The protests on Wednesday were reminiscent of those that followed the 2017 death of Philando Castile, who was sitting in his car after a traffic stop in a nearby suburb when an officer shot him.

Last year, in June, a young black man was killed by US marshals in Memphis, Tennessee, whose death sparked clashes with police overnight.

(With inputs from agency)