WASHINGTON, June 1 (CNA) - US President Donald Trump on Monday (Jun 1) said he was deploying thousands of "heavily armed" soldiers and police to prevent further protests in Washington, where buildings and monuments have been vandalised near the White House.

"What happened in the city last night was a total disgrace," he said during a nationwide address as tear gas went off and crowds protested in the streets nearby.

"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 denounced "acts of domestic terror" after nationwide protests against the death of an unarmed African American George Floyd in police custody devolved into days of violent race riots across the country.

"I want the organisers of this terror to be on notice that you will face severe criminal penalties and a lengthy sentence in jail," Trump said as police could be heard using tear gas and stun grenades to clear protestors just outside the White House.

He also called on state governors to "deploy the National Guard in sufficient numbers that we dominate the streets" before heading on foot for a photo op at the riot-damaged St John's, the two-century-old "church of the presidents" across from the White House.

He stopped in front of boarded-up windows at the yellow church, where many presidents have attended services, along with several members of his administration, including Attorney General William Barr, national security Adviser Robert O'Brien and other top aides.

As an acrid smell still hung in the air, Trump held up a Bible for cameras before walking back to the White House, but took no questions from reporters.

The president said in his White House remarks that he was mobilising all civilian and military resources "to stop the rioting and looting, to end the destruction and arson and to protect the rights of law-abiding Americans, including your Second Amendment rights" - a reference to the US constitutional protections for gun ownership.

"We cannot allow the righteous cries of peaceful protesters to be drowned out by an angry mob," Trump said, adding that the nation was gripped by "professional anarchists."

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."

The demonstrations, largely peaceful during the day but turning violent after dark, have erupted over the death of George Floyd, a 46-year-old African American who died in Minneapolis police custody after being pinned beneath a white officer's knee for nearly nine minutes.

A second autopsy ordered by Floyd's family and released on Monday found that his death was a homicide by "mechanical asphyxiation," meaning some physical force interfered with his oxygen supply. The report says three officers contributed to Floyd's death.

"The evidence is consistent with mechanical asphyxia as the cause of death, and homicide as the manner of death," Aleccia Wilson, a University of Michigan expert who examined his body at the family's request, told a news conference.

The Hennepin County Medical Examiner on Monday released details of its autopsy findings that also said Floyd's death was a homicide caused by asphyxiation.

The county report added that Floyd suffered cardiopulmonary arrest while being restrained by police and that he had arteriosclerotic and hypertensive heart disease, fentanyl intoxication and recent methamphetamine use.

The unrest has been the most widespread in the United States since 1968, when cities went up in flames over the slaying of civil rights icon Martin Luther King Jr, and rekindled memories of 1992 riots in Los Angeles after police were acquitted in the brutal beating of black motorist Rodney King.