MINNEAPOLIS, May 31 (CNA) -- : Minnesota's governor called up more national guard troops on Saturday (May 30) and warned they would "decimate" rioters after a fresh night of violence in Minneapolis overwhelmed law enforcement and protests against police brutality spread across America.

Officials in more than a dozen cities braced for more unrest as the protests sparked by the death of George Floyd in Minneapolis police hands this week sparked outrage over police mistreatment of African Americans.

It was one of the worst nights of civil unrest in the United States in decades.

Police cars and other property were set alight and destroyed in New York, Dallas, Atlanta and elsewhere, and protesters shouted and chanted for hours outside the White House in Washington, as President Donald Trump sat inside.

Governor Tim Walz said he was mobilising the entire 13,000-strong Minnesota National Guard to deal with the rioters who have looted shops and set fires in the Minneapolis-St. Paul area, a move not taken since World War II.

He also said he had spoken with the Pentagon on drawing on US military resources for peacekeeping, and The New York Times reported that, at Trump's bidding, military police were being put on notice for possible mobilisation, a step not taken in decades.

Walz said he would continue to support peaceful protests during the day as members of the African American community memorialised Floyd, whose death on Monday was seen on a horrifying cellphone video around the world.

But Walz alleged that protesters staying out past the 8pm curfew and wreaking "wanton destruction" of the city had nothing to do with Floyd.

"Our expectation is to have the curfew in place. Our expectation is to restore order," Walz said.

"It will be a dangerous situation on the streets tonight," he warned.

Saying the rioters were members of anarchist and white supremacist groups, he said: "Our goal is to decimate that force as quickly as possible."