Washington - President Donald Trump vowed Monday to order a military crackdown on once-in-a generation violent protests gripping the United States, saying he was sending thousands of 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 had been pinned down with a knee to the neck by a police officer -- 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 as police fired tear gas on protesters outside.

"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," he said.

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

After his address, protesters outside the White House were cleared so the president could walk across the street to the two-century old St John’s 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.

"What the president did today was he called out the American military against American citizens," New York governor Andrew Cuomo said on Twitter.

"He used the military to push out a peaceful protest so he could have a photo op at a church. It's all just a reality TV show for this president."

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

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

Many of the demonstrations have been peaceful and marked by cathartic moments such as officers hugging tearful protesters and marching or kneeling alongside them.