Hundreds of National Guard troops rolled into Philadelphia to help restore order Monday, even as new and frightening confrontations erupted between police and protesters on a third straight day of unrest.

Fires burned again, and the city remained under curfew following looting and destruction that overtook peaceful demonstrations sparked by the death of George Floyd at the hands of Minneapolis police.

A tired Mayor Jim Kenney declared that Philadelphia is “in the middle of one of the biggest crises in the city’s history,” simultaneously beset by civil unrest, a pandemic, economic devastation, and a primary election.

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Guardsmen wearing camouflage uniforms and carrying M4 Carbine rifles took up initial posts outside City Hall, the area of some of the worst losses, as large crowds grew and milled across Center City all day.

“Who do you protect? Who do you serve?” demonstrators chanted at a line of police officers on the Parkway.

On Monday evening, thousands of marchers moved onto the Vine Street Expressway, stopping traffic on the city’s major east-west thoroughfare. Some lay in the street.

Police fired tear gas into the crowd, setting off a blind, chaotic stampede as demonstrators tried to escape, scrambling up walls and steep hills and falling over one another.

“Tear gas! Tear gas!” people shouted as they ran and fell, many digging their hands into the dirt to pull themselves forward.

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People who reached the street-level top of the culvert were gassed again. Behind them lay a highway littered with bicycles and backpacks, all left behind as people ran.

News footage soon showed at least 30 demonstrators in handcuffs.

U.S. Attorney William McSwain threatened to bring federal charges against people who riot or loot in the Philadelphia area, saying “it is time to confront any continued violence and stop it in its tracks.”

McSwain said the unrest “endangers the lives and the livelihoods of others and interferes with the rights of peaceful protesters, as well as the rights of other citizens of this city. It also undercuts the work that needs to be done to address people’s legitimate grievances.”

The cry of anger, sorrow, and frustration echoed across the city on Monday.

“I have been grieving my entire life,” an African American speaker told thousands of sign-waving, fist-pumping protesters outside of Philadelphia Police Headquarters. “Stop killing us.”

The coronavirus crisis suddenly seemed distant, even as it continued to kill and sicken. More than 104,000 Americans have now died.

“The pandemic has not been canceled,” said Rep. Brendan Boyle (D., Phila.). “The virus COVID-19 still exists. It doesn’t know or care about the sort of tumult that the United States of America is involved in over the last several days.”

Protesters ignored that danger, gathering in groups of hundreds and thousands at places across the city. Many wore protective masks, others did not.

At midday, National Guard and Philadelphia police barricaded JFK Boulevard. Crowds gathered on the south side of City Hall near the statue of Octavius Catto, the great civil-rights leader. A group of city schoolteachers handed out bottled water and snacks to protesters near Police Headquarters, imploring people to “keep the peace.”

Four Philadelphia police officers near City Hill joined demonstrators in taking a knee, a gesture that’s become a universal sign of racial and social solidarity against oppression.

Early on Monday, there were signs that the city was struggling to its feet, even as looting and violence spilled into suburban counties.

Upper Merion instituted a curfew after 12 people were arrested late Saturday while trying to break into the King of Prussia Mall. Upper Darby did the same after looting and vandalism near its border with West Philadelphia. Police in Bensalem Township, adjacent to Northeast Philadelphia, assisted city officers at the Philadelphia Mills Mall, then rushed to the Neshaminy Mall after reports of a break-in at the Sears store.

“Thank God criminals are not that smart,” said Bensalem director of public safety Frederick Harran. “To their surprise, they were met with an empty store, because Sears has been closed for about two years.”

At least 17 were arrested in Atlantic City during widespread looting, and a 7 p.m. curfew was to remain in place there until June 8.

Police authorities across 43 cities have made more than 7,200 arrests for charges including burglary, arson, aggravated assault, rioting, and looting, according to data collected by CBS News. In south Minneapolis on Monday, an emotional Terrence Floyd went to the intersection where his brother died and asked for peace.

He told those gathered around him that “in every case of police brutality the same thing has been happening. You have protests, you destroy stuff. ... Let’s do this another way.”

He struggled for composure — his brother died in handcuffs after a white officer knelt on his neck for more than eight minutes — then declared, “My family is a peaceful family,” words that seemed to echo the prophet Isaiah, who said, “My people will dwell in a peaceful place, in safe and secure places of rest.”

At the White House on Monday evening, President Donald Trump declared himself “your president of law and order,” declaring he would deploy “thousands and thousands of heavily armed soldiers, military personnel, and law enforcement officers” to bring order.

Earlier he castigated the nation’s governors as “weak” during a video teleconference, insisting that cities like Philadelphia, New York, and Los Angeles must mimic Minneapolis, where thousands of National Guard members joined police in swarming the streets over the weekend.

“No domination. You have to dominate,” Trump said, according to audio obtained by CNN. “Most of you are weak. You have to arrest people.”

Presumptive Democratic presidential nominee Joe Biden said he would speak in Philadelphia on Tuesday, though specific plans were still developing.

Philadelphia police say at least 429 people have been arrested so far, and at least 18 officers have been injured with two hospitalized. The Fire Department responded to about 250 fires across the city on Sunday.

“What you’re seeing across the country right now is unlike any of us have seen. We have been sitting on a powder keg for quite some time and it has burst,” said Police Commissioner Danielle Outlaw.

In Center City, the day began calmly, the dominant soundtrack the high whine of circular saws, as business owners and others cut sheets of plywood to cover broken windows.

At Washington Square, the hallowed Tomb of the Unknown Solider of the American Revolution was defaced by spray paint. Vandals wrote Committed Genocide and ACAB — meaning, All Cops Are Bastards — on the granite wall behind Jean-Antoine Houdon’s bronze statue of Washington, the monument’s centerpiece.

In West Philadelphia, the air was clear, different from the night before, when tear gas wafted after police said people threw bricks, rocks, and Molotov cocktails while setting cruisers ablaze. Residents around 52nd Street were outside on Monday with brooms and garbage cans, trying to clean up.

“These things can be replaced,” said Marietta Wesley, “but we’re coming together.”

David Thompson, 50, was at Market and 52nd trying to get a bus home on Monday evening. He said the impact of the looting is being felt by residents most in need.

“The saddest part that came to reality today was — it’s the first of the month. And a lot of people had to go shopping, and they couldn’t go shopping because all the shopping places are closed, from ShopRite to Rite Aid. ... A friend of mine, along with other people that I know, they couldn’t get their medications.”

Near City Hall on Monday night, a heavy police and Guard presence was in place as the city’s 6 p.m. curfew thinned the crowds. A group of about 12 held a sit-in.

“We meet you with peace,” a protest speaker said. “But you meet us with the Army.”

A police officer told them, "You don’t move in two minutes, you’re getting locked up.”

People rose, hugged, and said goodbye, disbanding for the night.

Staff writers Wendy Ruderman, Jeremy Roebuck, Vinny Vella, Elizabeth Robertson, Pranshu Verma, Anna Orso, Allison Steele, and Justine McDaniel contributed to this article, as did the Associated Press.