In Philadelphia on Sunday, protesters ransacked stores and set fires. City police said they had made 215 arrests as of Sunday afternoon.

"Deeply saddened by the destruction we saw last night in Center City. In looting downtown, these individuals not only desecrated private businesses, they also desecrated the important message that was heard in the earlier, peaceful protests," Mayor Jim Kenney said on Twitter on Sunday.

The city has announced a curfew between 6 pm and 6 am after a day of protests and looting.

In the New York City borough of Brooklyn, a patrol car being pelted with projectiles on Saturday slowly accelerated through a group of protesters, and Mayor Bill de Blasio called for an investigation of the incident. Also in New York, an officer was recorded roughly shoving a young woman to the pavement after she had approached a phalanx of police, and another was shown lifting the mask of a protester and pepper-spraying him.

In two separate incidents in Brooklyn over the weekend, two lawyers and one young woman were charged with throwing improvised gasoline bombs at police vehicles.

The New York Police Department made 350 arrests on Saturday, while 30 officers were injured. Graffiti was scrawled on St Patrick's Cathedral in Manhattan. Protesters also marched in Times Square on Sunday.

The nationwide unrest started after George Floyd, 46, died on May 25 when a Minneapolis police officer knelt on his neck for almost nine minutes. The officer, Derek Chauvin, has been charged with third-degree murder and manslaughter. He and three other city officers were fired shortly after the incident, which started when Floyd was accused of trying to pass a counterfeit bill at a store.

Trump, who has mostly taken a hard line against the protests in several posts on Twitter, declared on Sunday that a left-wing movement of different groups called antifa was a domestic terrorist organization. Antifa, short for "anti-fascist", was accused by US Attorney General William Barr of infiltrating the protests in various cities and fomenting disorder.

"We are a nation in pain right now, but we must not allow this pain to destroy us," former vice-president Joe Biden, the presumptive Democratic nominee to face Trump in November's election, tweeted on Sunday. "As President, I will help lead this conversation－and more importantly, I will listen, just as I did today visiting the site of last night's protests in Wilmington."

US House Speaker Nancy Pelosi, appearing on ABC's This Week on Sunday, said: "Democratic and Republican presidents alike have seen it as their responsibility to be a unifying force in our country, not to fuel the flame. President Trump has made clear he would rather distract than perform his duty."

Other major US cities where clashes intensified over the weekend included Atlanta, where CNN's headquarters were attacked, Chicago, Dallas, Houston, Los Angeles and Washington.

Houston Mayor Sylvester Turner said on Sunday that Floyd's body will be returned to the city.

"This is the same city that George Floyd grew up in, and his body will be returning to this city," Turner said at a news conference. "And so the focus needs to be on supporting and uplifting his family."

May Zhou in Houston, Liu Xuan and Zhou Jin in Beijing, Xinhua and agencies contributed to this story.