Washington, May 29 (IANS): Minnesota Governor Tim Walz has declared a state of emergency and activated the National Guard to restore order after protests over the death of a black man in police custody turned violent, with widespread damage and looting in Minneapolis city.

George Floyd died on Monday evening in Minneapolis shortly after a white police officer held him down with a knee on his neck though the victim repeatedly pleaded, "please, I can't breathe" and "don't kill me", reports Xinhua news agency.

The four officers involved in the case were fired shortly after a video showing Floyd's death went viral on social media on Tuesday, sparking a national outcry for justice.

Walz's order came after Minneapolis Mayor Jacob Frey's called for assistance after widespread damage and looting took place amid the protests on Wednesday night.

"George Floyd's death should lead to justice and systemic change, not more death and destruction," Walz said in a statement.

"It is time to rebuild. Rebuild the city, rebuild our justice system, and rebuild the relationship between law enforcement and those they're charged to protect."

The National Guard Adjutant General will work with local government agencies to provide personnel, equipment, and facilities needed to respond to and recover from this emergency, according to a report from local newspaper Star Tribune.

In addition, the Minnesota State Patrol will supply 200 troopers. State Patrol helicopters and other aircraft will also be available to assist law enforcement officers on the ground, said the report.

Wednesday night's protests began peacefully and escalated into gunfire and arson, with about 30 buildings reported ablaze, and countless others damaged, said the report.

Amid national outrage over the incident, House Speaker Nancy Pelosi said on Thursday that Floyd was "murdered" at the hands of the Minneapolis Police Department.

"We saw it on TV - him being murdered on TV," Pelosi told reporters in the Capitol.

Appearing on CNN later in the day, the top Democrat called Floyd's death "an execution".

"We saw it, so we can't deny it or qualify it," she said, adding: "So justice has to be done."

House Democrats are eyeing a variety of responses, including the adoption of more stringent federal policing standards and a pressure campaign on the Justice Department to get more aggressive in tackling the issue of police misconduct targeting African-Americans, according to a report of The Hill.

"There are all kinds of expressions of concern," Pelosi said. "But not just expressions of concern, plans to take action so that this stops."

Besides Minneapolis, protests against the incident took place in some other US cities including Los Angeles and Memphis, local media reported.

The Justice Department said on Thursday it will make the case a "top priority".