Widespread looting erupted in Manhattan’s central business district, long a symbol of New York City’s prominence, with assaults on some of the city’s best-known retailers.

In episodes that began Monday afternoon and grew wilder as night fell, small bands of young people dressed mostly in black pillaged chain stores, upscale boutiques and kitschy trinket stores in Midtown Manhattan, as the police at first struggled in vain to impose order.

Macy’s flagship store in Herald Square, which had been shuttered like all other nonessential businesses in the city because of the coronavirus, was ransacked, and several luxury stores along Fifth Avenue were looted.

Despite doubling the number of officers deployed to 8,000, the police struggled to respond to reports of stores under assault across the city. Nike, Anthropologie, Aldo, a New York Yankees store and two Rolex watch shops were among those targeted.

Mayor Bill de Blasio acknowledged that the Monday night curfew, announced several hours before it was to begin, had failed to quell the violence that marred the peaceful protests of previous days. As a result, he said, a curfew would be imposed again on Tuesday, this time starting three hours earlier, at 8 p.m.

“We’re seeing too much of this activity tonight,” he said in an interview on NY1, a local television station.

With curfews imposed in dozens of U.S. cities over the weekend, the measure is particularly striking for New York City’s eight million residents, who have been under strict lockdown orders because of the coronavirus pandemic, which has killed thousands of city residents.

With the city preparing to gradually reopen on June 8, the protests have injected a new level of unease, coming with not only police confrontations and widespread looting, but also fears that the coronavirus is spreading in the crowds.

As President Trump delivered a speech in the Rose Garden on Monday in which he called himself “an ally of all peaceful protesters” yet vowed to send the military to states where governors could not bring rioting under control, the sound of explosions and screams could be heard in the distance.

In his first remarks from the White House since huge protests have swept the country, he called the looting and violent demonstrations “acts of domestic terror.”

Mr. Trump said he was among those “rightly sickened and revolted” by the death of George Floyd. But the president said that “if a city or state refuses to take the actions 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.”

Just after he concluded his speech, military police from the National Guard clad in camouflage and riot shields surged in front of a line of law enforcement officers pushing protesters back from the mouth of Lafayette Square outside the White House.

Officers used tear gas and flash grenades to clear out the crowd so that Mr. Trump could visit the nearby St. John’s Church, the site of a parish house basement fire on Sunday night. He stood in front of the boarded-up church to pose for photographs with a Bible.

Bishop Mariann Edgar Budde of the Episcopal Diocese of Washington said she was “outraged” that Mr. Trump had gone to the church “after he threatened to basically rain down military force.”

Military helicopters also performed a “show of force” maneuver that is often used in combat zones to scare away insurgents. The crowd quickly dispersed into surrounding blocks. Minutes later, the Black Hawk returned for another pass.

Rewind, before the trash fires, lootings and arrests, to the scene outside Brooklyn’s Barclays Center on Sunday evening.

A woman who works as a Kennedy Airport gate agent stood in the crowd with the setting sun at her back, thinking about the time the police had hassled her little brother. Several feet away, a young media consultant handed out free pizza and water with friends at a table that one of them had brought along. A 40-year-old man held his phone aloft, sharing the scene with a longtime friend — the two were once beaten by the police as teenagers, he said.

Soon the group would march through the area’s broad avenues and narrow side streets, greeted with applause, honking horns and raised-fist salutes. Bryce Stewart, 35, of Bushwick, stopped his motorcycle and climbed atop it for a better look.

“This is beautiful,” he said.

That mood — of spirited, sometimes vulgar, but essentially peaceful indignation — lasted until dark. Then, as it had on each previous night of protest, the glass started to shatter.

It began Sunday around 10 p.m. in SoHo, when a knot of young men on the periphery of a large march from Brooklyn smashed a clothing store window and stole a jacket, dragging a mannequin out onto the sidewalk.

It was a pattern that repeated itself across the nation again on Monday.

Rick Rojas in Hattiesburg, Miss.

Rebekah Castilaw stood on an island of grass along one of the city’s busiest thoroughfares — a protest of one.

She had brought a few signs with her, and the one she was holding at the moment had “Black Lives Matter” handwritten in black marker. As cars whipped past her, many drivers honked. Plenty of people rolled down their windows. Most cheered her on, and some hurled vulgarities.

“I’ll be out here every day,” Ms. Castilaw, who is white, said from her spot outside the University of Southern Mississippi. “It’s pitiful you don’t see more.”

Dionne Searcey in Minneapolis

About 200 people turned out in the suburban Minneapolis community of Maple Grove for a candlelight vigil. Their plan was to stand in silence for eight minutes and 46 seconds, the amount of time that the Minneapolis police officer had his knee on George Floyd’s neck.

But even after the time elapsed, people remained frozen in place for several minutes, with only the sounds of chirping birds and the hum of cars on the freeway behind them. Members of the mostly white crowd included children on scooters and bicycles, and they held fists in the air and carried “Black Lives Matter” posters as they stood along the freshly cut lawn of the town library’s driveway.

Mary Kriz, leaning against her bicycle, said she was outraged by President Trump’s message on Monday about using military force to break up protests. “It couldn’t be a more wrong solution,” she said. “What we’re hearing from George’s family is they want us to protest peacefully. It’s the worst possible solution.”

John Morrisette, who was clutching a candle and standing beside her, nodded and said, “Everybody is looking for peace right now, and war is not the answer.”

As a curfew approached, members of the National Guard appeared to be rolling into position in the part of town where Mr. Floyd died. More than half a dozen troops stood outside Chumps Chicken and the Cedar Bar & Grill, flanked by an array of armored vehicles. A few blocks away, a convoy of military vehicles and police vehicles with sirens flashing passed down the street.

Adam Popescu in Los Angeles

Just as the sun set and a citywide curfew took effect, a huge gathering marched down the iconic Hollywood and Sunset Boulevards.

Police officers gave the group a wide berth, and things initially seemed peaceful. But soon groups peeled off, targeting the Gower Gulch shopping complex, smashing the windows of a kebab restaurant, and tearing the plywood barriers from a drugstore and a mobile phone shop.

Then the frenzy began. People wearing masks stormed the stores, their arms heavy with looted goods.

Officers quickly descended on the scene, but just as quickly, many demonstrators jumped into awaiting vehicles and fled.

Zolan Kanno-Youngs in Washington

“It’s been the last straw,” Janasia Crumpler, 20, said at a rally. “It’s a pandemic. I’m in good health. I came out for those who can’t and people who’ve been marching for 50 years.”

Ms. Crumpler said that like many people her age, she had first expressed her outrage on social media. But she was then compelled to take to the streets by the contrast in the government’s response to armed white demonstrators storming state capitals to protest coronavirus restrictions.

“It was out of control before when white people were rioting and you called them very good people — and they were rioting about the virus,” Ms. Crumpler said as she walked.

An American flag burned in the street nearby.

‘In every city there’s a George Floyd.’ Voices from the street.

For a week, cities across America have been theaters of dissent.

The people expressing their anger and frustration are individual pieces of a movement, like drops of water to a wave. Their strength is in cohesiveness. Yet they are strangers, divided by geography, age, color and experience.

Here are some of those making their voices heard.

Kennetta Hollivay, 49, Minneapolis

Kennetta Hollivay stood outside her store, the Dollar & Up market, a block and a half from the spot where George Floyd died. She and her husband had bought the store in September, and although the store remained open through the pandemic, business was slow.

Ms. Hollivay, who has lived in the neighborhood her whole life, said she had felt compelled to join the protests, at least initially.

“When I first heard about it, I was like, ‘Oh, wow, the police have killed somebody else,’” she said. “And I was hurt. But once I saw video, it was like — that man died right before our eyes. I’ve never seen nothing like that before. Ever. Ever. I told my husband yesterday I’ve been having these dreams every night of this. Nightmares.”

Chad Bennett, 22, St. Louis

Chad Bennett and his father, wearing matching face masks, stood back in a parking lot as they watched protesters march past the Police Department in Ferguson, Mo., the site of numerous protests since Michael Brown, a black teenager, was killed by a white police officer there in 2014.

“When Ferguson happened, the whole world descended on us,” said Mr. Bennett, a graduate of Columbia College Chicago who works as an animator. “This time, it was like bam, bam, bam, city after city. I knew I had to be a part of it.”

Seeing the video of what happened to Mr. Floyd left him “numb,” he said. “It’s a silent rage, I guess,” he said. “I don’t know if I’m sad anymore. I’m just angry.”

Beth Muffett, 36, Austin, Texas

For most of her life, Beth Muffett, a stay-at-home mother and massage therapist, had positive interactions with law enforcement.

But by the time she and her friends had left a protest outside City Hall on Sunday, she had bruises on her stomach and knee from where one officer had struck her with his bicycle, and another bruise on her arm after she had fallen back onto another protester.

“There’s a lot of privileged white women, and I’m one of them,” Ms. Muffett said. “I’ve never had a cop treat me like that.”

On #BlackoutTuesday, artists go quiet to focus attention on protesters’ message.

Millions of people worldwide are heeding a call for a day of silence on social media to amplify black people’s voices under the hashtag #BlackoutTuesday.

The idea, which came in response to the death of George Floyd, Breonna Taylor, Ahmaud Arbery and others, began as a movement within the music industry as a campaign organized by Jamila Thomas and Brianna Agyemang, called “The Show Must Be Paused,” was amplified by several major record labels.

And when stars like Rihanna, the Rolling Stones, Drake, Quincy Jones and Billie Eilish shared the idea to their millions of followers, the idea took off. By early Tuesday, more than two million Instagram featured the hashtag #BlackoutTuesday, and hundreds of thousands tagged #TheShowMustBePaused.

Radio shows and music channels pledged to “black out” for the day, and the streaming platform Spotify said it would add eight minutes and 46 seconds of silence to some playlists and podcasts to echo the length of time that a Minneapolis police officer knelt on George Floyd’s neck.

Theaters and actors have joined in, pledging to mute themselves and instead listen. And bloggers and influencers likewise grasped hold of the initiative, with many replacing their profile photos with a black circle or adding a photo of a plain black square within their feed.

People with smaller followings are also letting their social media accounts go dark for the day, sharing posts from black people and donating to organizations that work to fight racism.

Around the world, demonstrators expressed solidarity with U.S. protesters.

Thousands gathered for a march in Sydney, Australia, on Tuesday, and chanted “Enough is enough” while kneeling outside the U.S. Consulate building — the latest in a series of peaceful global rallies as the U.S. protests have resonated around the world.

In places like London and Rio de Janeiro, demonstrations of solidarity with those taking to the streets in America have also prompted soul searching over racial divisions in their own nations.

The issue of police brutality has particular resonance in Rio de Janeiro, where more than 1,400 people were killed by the police from January to September of last year, and where law enforcement officers have been accused of widespread abuse, according to Human Rights Watch. On Sunday, thousands there gathered and echoed George Floyd’s dying words, “I can’t breathe,” before the crowd was dispersed with tear gas.

In Paris, dozens knelt outside the U.S. Embassy, most of them dressed in black and some holding signs that read, “Racism is choking us.” In a statement before the event, organizers pointed to recent police violence against people of color in their own community and demanded that those acts be met with “the greatest firmness in France.”

On Monday evening, demonstrators in Berlin rallied at the Brandenburg Gate and thousands gathered in Dublin to march to the U.S. Embassy.

“Black lives in America seem to be taken for granted,” one protester told Ireland’s national broadcaster, RTE.

In New Zealand, Prime Minister Jacinda Ardern said that while she was “horrified” by Mr. Floyd’s death, protests held in Wellington on Monday were a “clear breach” of a ban on public gatherings during the coronavirus pandemic.

“We need to show that and express that solidarity in a way that makes sure that we are looking after one another as well,” she said.

Reporting was contributed by Mike Baker, Kim Barker, Julie Bosman, John Branch, Kimiko de Freytas-Tamura, Caitlin Dickerson, John Eligon, Tess Felder, Manny Fernandez, Thomas Fuller, Russell Goldman, Miriam Jordan, Zolan Kanno-Youngs, David Montgomery, Jack Nicas, Elian Peltier, Adam Popescu, Austin Ramzy, Frances Robles, Rick Rojas, Marc Santora, Anna Schaverien, Dionne Searcey, Megan Specia, Daniel Victor and Neil Vigdor.