Protesters have clashed with police in cities across the US over the killing of an unarmed African-American man at the hands of officers in Minneapolis.
Minnesota's governor said the tragedy of the death of George Floyd in police custody had morphed into "something much different - wanton destruction".
New York, Atlanta, Portland and other cities have seen violence, while the White House was briefly locked down.
An ex-Minneapolis policeman has been charged with murder over the death.
Derek Chauvin, who is white, was shown in footage kneeling on 46-year-old Mr Floyd's neck on Monday. He and three other officers have since been sacked.
Mr Chauvin, 44, is due to appear in court in Minneapolis for the first time on Monday.
President Donald Trump has described the incident as "a terrible, terrible thing" and said he had spoken with Mr Floyd's family, whom he described as "terrific people".
The Floyd case has reignited US anger over police killings of black Americans, and reopened deep wounds over racial inequality across the nation.
It follows the deaths of Michael Brown, Eric Garner and others, which have all occurred since the Black Lives Matter movement was sparked by the acquittal of neighbourhood watchman George Zimmerman in the shooting death of Trayvon Martin in 2012.
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 with fire officials unable to reach some sites.
Television pictures also showed looting in Minneapolis, with police officers thin on the ground.
Only at about midnight (05:00 GMT) did police and National Guard troops move in in any numbers, the Star Tribune reports.
State Governor Tim Walz, in an early morning press briefing, described the situation as "chaotic, dangerous and unprecedented".
He said he took responsibility for "underestimating the wanton destruction and the sheer size of this crowd" when questioned about the lack of police on the streets.
He said the Guard deployment was the largest in state history but admitted "there's simply more of them than us". He said those on the streets "don't give one damn" about the stay-at-home order.
The Pentagon has put the military on alert for possible deployment in Minneapolis.
On Friday evening, crowds gathered near the White House in Washington waving photographs of Mr Floyd and chanting "I can't breathe" - invoking his last words and those of Eric Garner, a black man who died after being held in a police chokehold in New York in 2014.
The White House was then temporarily placed on lockdown, with the US Secret Service closing entrances and exits.
In Atlanta, a state of emergency was declared for some areas to protect people and property. Buildings were vandalised and a police vehicle was set alight as protesters gathered near the offices of news broadcaster CNN.
Mayor Keisha Lance Bottoms issued a passionate 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.
Mayor Bill de Blasio tweeted: "We don't ever want to see another night like this."
The mayor of Portland, Oregon, has declared a state of emergency amid looting, fires and an attack on a police precinct. An immediate curfew until 06:00 local time (13:00 GMT) was imposed and it will restart at 20:00.\
In Detroit, police are investigating after a 19-year-old man was killed when a vehicle pulled up to protesters and shots were fired into the crowd.
In Dallas, officers launched tear gas canisters after they were pelted with stones, with tear gas also fired in Phoenix, Indianapolis and Denver.
Protesters blocked roads in Los Angeles and also in Oakland, where windows were smashed and "Kill Cops" graffiti sprayed.