Violent protests over the death of an unarmed African-American man at the hands of police officers in Minnesota "are no longer in any way" about his killing, the state's governor has said.
Tim Walz spoke after a night of unrest in several US cities over the death of George Floyd in Minneapolis.
He said that as a result he was taking the unprecedented step of mobilising the state's entire National Guard.
There are also reports that military police units could deploy.
An ex-Minneapolis policeman has been charged with murder over the death of Mr Floyd, 46, in police custody on Monday.
Derek Chauvin, 44 and white, was shown in footage kneeling on Mr Floyd's neck for several minutes, even after he said he couldn't breathe. He and three other officers have since been sacked.
The video went viral online, reigniting US anger over police killings of black Americans, and reopening deep wounds over racial inequality across the nation.
On Friday night protesters clashed with police in cities including New York, Atlanta and Portland. In Washington DC, the White House was briefly locked down.
In Minnesota, "our great cities of Minneapolis and St. Paul are under assault," Mr Walz said. "The situation in in Minneapolis is now about attacking civil society, instilling fear and disrupting our great city."
He said violence on Friday night had made "a mockery of pretending this is about George Floyd's death, or inequities or historical traumas to our communities of colour".
He and other officials have suggested that many violent protesters came from outside the state, but did not give details.
President Donald Trump praised the US Secret Service for securing the White House but said if protesters breached the perimeter they "would have been greeted with the most vicious dogs, and most ominous weapons, I have ever seen". He blamed violence on "organised groups".
What's the latest on the protests?
Minnesota remains the most volatile region, with curfews ordered for the Twin Cities of Minneapolis-Saint Paul from 20:00 to 06:00 on Friday and Saturday evening.
Protesters defied the curfew on Friday. Fires, many from burning cars, were visible in a number of areas and television pictures showed widespread looting.
About 500 National Guard troops moved in and Mr Walz said he was deploying 1,000 more for Saturday night.
There are long-standing tensions between the black community and police in Minneapolis, with another black man, Philando Castile, shot and killed in 2016 during a traffic stop close to the epicentre of the protests.
African-Americans also suffer significant socio-economic inequalities, entrenched segregation and have a higher unemployment level that the white community.
In Atlanta, Georgia, where buildings were vandalised, a state of emergency was declared for some areas to protect people and property.
Mayor Keisha Lance Bottoms issued an impassioned plea, saying: "This is not a protest. This is not in the spirit of Martin Luther King Jr. You are disgracing our city. You are disgracing the life of George Floyd."
In New York's Brooklyn district, protesters clashed with police, throwing projectiles, starting fires and destroying police vehicles. A number of officers were injured and many arrests made.
The mayor of Portland, Oregon, has declared a state of emergency amid looting, fires and an attack on a police precinct.