The police monitor a protest over the death of George Floyd on Times Square in New York, the United States, June 1, 2020. (Xinhua/Wang Ying)

US President Donald Trump vowed on Monday to order a military crackdown on the once-in-a-generation violent protests gripping the US, saying he was sending troops onto the streets of the capital and threatening to deploy soldiers to states unable to regain control.

The dramatic escalation came a week after the death in Minneapolis of George Floyd, an unarmed black man who was killed when a white police officer knelt on his neck, leading to the worst civil unrest in decades in New York, Los Angeles and dozens of other American cities. 

After being criticized for his silence on the worsening crisis, Trump struck a martial tone in a nationwide address from the White House garden, as police fired tear gas on peaceful protesters outside the fence.

"I am dispatching thousands and thousands of heavily armed soldiers, military personnel and law enforcement officers to stop the rioting, looting, vandalism, assaults and the wanton destruction of property," Trump said.

He slammed the previous night's unrest in Washington as a "total disgrace" and called on governors to "dominate the streets." 

"If a city or state refuses to take the actions that are necessary to defend the life and property of their residents, then I will deploy the United States military and quickly solve the problem for them," he said, denouncing "acts of domestic terror."

Despite the president's rhetoric, Monday's protests appeared largely peaceful in major cities, though some looting was reported in New York and Los Angeles. 

During his address, however, law enforcement including military police used tear gas to clear protesters outside the White House so the president could walk across the street to the two-centuries-old St Johns church, hit with graffiti and partially damaged by fire during unrest on Sunday.

"We have a great country," Trump declared as he stood before the church's boarded up windows, held up a Bible and posed for photographs. 

The backlash was swift.

"He's using the American military against the American people," tweeted Democratic presidential hopeful Joe Biden.

Thousands of people have participated in the nationwide demonstrations against police brutality and racism since Floyd's killing.

It was the most widespread unrest in the US since 1968, when cities went up in flames over the slaying of civil rights icon Martin Luther King Jr.

Monday, Zimbabwe summoned the US ambassador over remarks by a senior American official accusing the southern African country of stirring up anti-racism protests over the death of George Floyd, the government said.

Zimbabwe's Foreign Minister Sibusiso Moyo dismissed as "false and without factual foundation" the claims made by Trump's national security adviser, Robert O'Brien.

AFP