People hold signs and protest during a rally after a Minneapolis Police Department officer allegedly killed George Floyd on Tuesday in Minneapolis, Minnesota. A video of a handcuffed black man dying while a Minneapolis officer knelt on his neck for more than five minutes sparked fresh furor in the US over police treatment of African Americans. Photo: AFP

Minnesota Governor Tim Walz on Thursday declared a state of emergency and activated Minnesota National Guard to restore order after protests over the death of George Floyd in police custody turned violent last night with fires burning and businesses looted in Minneapolis, the biggest city in the midwest state.

Floyd died on Monday evening shortly after a white police officer held him down with a knee on his neck though the black man in his 40s repeatedly pleaded, "I can't breathe," and "please, I can't breathe." The police officer's way of handling the man is not approved by the local police department.

Walz's order came after Minneapolis Mayor Jacob Frey's calls for assistance after widespread damage and looting took place amid the riot 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," said the governor.

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.