WASHINGTON – Crowds protesting the killing of George Floyd clashed with U.S. Secret Service and Park Police officers in the nation’s capital Saturday afternoon and evening, the second outburst of violent confrontations in less than 24 hours between federal law enforcement and activists decrying police brutality.

By nightfall, nearly 1,000 protesters were circling the perimeter of the White House grounds, which was fortified with law enforcement vehicles, metal barriers and rows of armored Secret Service, District of Columbia police and U.S. Park Police.

Sweating, packed closely together and shouting through masks worn to protect themselves from the deadly coronavirus still consuming the Washington region, the protesters launched fireworks and threw bottles at the officers, who swung batons and fired pepper-spray projectiles to push them back. As the sun began to set, District National Guard trucks rumbled through the streets.

The confrontations came after an extraordinary scene outside the White House in the early hours of Saturday captured the anger and divisions that have gripped the nation. More than 1,000 demonstrators massed along Pennsylvania Avenue, throwing bricks and rocks and dispersing only after 3 a.m., when the Secret Service began to fire chemical agents. No similar scene has unfolded within view of the North Portico of the president’s home in recent memory.

By Saturday night, that scene was unfolding again. Lafayette Square – the park in front of the White House where tourists can ordinarily be seen strolling around statues of Andrew Jackson and a suite of Revolutionary War heroes – was walled off with metal barricades, another symbol of America’s dramatic departure from the normal course of civic life.

At the north edge of the park, police repeatedly clashed with a dense pack of protesters for whom the “social distancing” practiced through months of a pandemic seemed a distant memory.

“Hey hey, ho ho, you racist cops have got to go,” the crowd chanted.

“Just put the shield down. Come join us,” one woman said. “This is the right side.”

A young man next to her shouted, “How will your grandchildren remember you?”

Nearby, a 30-year-old District resident who would only give his first name, Phil, raised an American flag above the crowd. As he waved, those around him began to chant: “Burn it!”

The second round of clashes erupted just hours after President Donald Trump, responding to the unrest in Washington, made a string of inflammatory statements on Twitter. He warned protesters they would be attacked by “vicious dogs” if they breached the White House grounds, falsely accusing District Mayor Muriel Bowser, a Democrat, of refusing to let her city’s police force help quell the demonstrations, and appeared to invite his supporters to clash with protesters in the streets Saturday night.

On social media and at an afternoon news conference, Bowser said the president had “glorified violence” with his comments, saying they were “an attack on humanity and an attack on America, and they make our city less safe.” She said his Twitter posts harked back to some of the darkest scenes of the civil rights movement, when police unleashed attack dogs on peaceful black demonstrators.

“I’m just shaken that an American president would utter such words about fellow Americans,” Bowser said.

It was an unusually visceral statement from a mayor who typically exercises restraint in responding to the president’s periodic baiting of District government officials. Bowser and District Police Chief Peter Newsham said city police had in fact joined with Secret Service and U.S. Park Police on Saturday to control the demonstrators on Pennsylvania Avenue, even supplying the Secret Service with extra protective gear.

The Secret Service issued a statement that appeared to confirm their account, stating, “The Metropolitan Police Department and the U.S. Park Police were on the scene.” Washington Post reporters at the scene of the protest early Saturday morning observed District police present.

Bowser also issued a sharp rebuttal of the president’s comments on Twitter.

“While he hides behind his fence afraid/alone, I stand w/ people peacefully exercising their First Amendment Right after the murder of #GeorgeFloyd & hundreds of years of institutional racism,” the mayor wrote of Trump. “There are no vicious dogs & ominous weapons. There is just a scared man. Afraid/alone.”

Newsham and Bowser said they are prepared for additional demonstrations over the weekend. Bowser said the Park Police has requested assistance from the National Guard if needed for “crowd control.” Later Saturday, Guard trucks were seen near the Capitol on the way to deployment.

Demonstrations began again in the District on Saturday morning as a few dozen protesters met outside the White House. They included Jerry Collins, a 74-year-old deacon at Holy Family parish in Hillcrest Heights, Maryland.

Collins said he felt encouraged by the attention on Floyd’s death and other cases of institutional racism. “I hope the groundswell maintains,” he said. “It says people want change.”

By the afternoon, the number of protesters swelled to the hundreds, starting at the U.S. Capitol, many shouting, “Let us breathe, let us breathe.” A car caravan organized by Black Lives Matter protesters also rolled across the city as demonstrators gathered downtown.

The protests in the District were among those in many cities nationwide. The officer who put his knee on Floyd’s neck for nearly nine minutes has been charged with third-degree murder and manslaughter.

In Richmond, crowds damaged buildings and state offices surrounding the Virginia State Capitol on Friday, prompting officials on Saturday to close Capitol Square until further notice.

A window was broken in the Barbara Johns Building, which houses the offices of the state attorney general. Also vandalized were the visitor’s entrance to the state capitol, the Virginia Supreme Court building and the Washington Building, which houses several state offices.

Friday’s demonstrations in the District began about 5 p.m., with a gathering of several hundred people. Newsham said that group quickly grew to more than 1,000 people, who marched south to the White House. A few skirmishes occurred there before the group marched down Pennsylvania Avenue to the U.S. Capitol and continued on its way. A group returned to the White House about 11:30 p.m.

The District’s Metropolitan Police Department, which has nearly 4,000 sworn officers, routinely coordinates closely with the Secret Service and with other federal law enforcement agencies that help protect the nation’s capital, including the Capitol Police and the Park Police. The agencies handle many demonstrations each year.

Authorities said bricks, fireworks and water bottles were thrown at officers, and some demonstrators sprayed mace. At least one demonstrator was able to grab a riot shield from Secret Service officers. Others tore bricks from the street and broke them into smaller pieces.

The Secret Service said in its statement that six people were arrested. “Demonstrators repeatedly attempted to knock over security barriers on Pennsylvania Avenue,” the statement says. “No individuals crossed the White House Fence and no Secret Service protectees were ever in any danger.”

At one point, demonstrators were able to wrest some of the metal barricades away from officers. Police issued two warnings to disperse at 3:30 a.m. before the line of officers with shields advanced through the park, some firing chemical spray.

Trump responded with a number of tweets on Saturday morning. “Great job last night at the White House by the U.S. @SecretService. They were not only totally professional, but very cool,” he began.

The president added, “They let the ‘protesters’ scream & rant as much as they wanted, but whenever someone . . . got too frisky or out of line, they would quickly come down on them, hard – didn’t know what hit them. The front line was replaced with fresh agents, like magic. Big crowd, professionally organized, but nobody came close to breaching the fence.

“If they had they would . . . have been greeted with the most vicious dogs, and most ominous weapons, I have ever seen. That’s when people would have been really badly hurt, at least. Many Secret Service agents just waiting for action. ‘We put the young ones on the front line, sir, they love it, and . . . good practice.’ “

About a half-hour later, Trump tweeted again, this time taking aim at the demonstrators: “The professionally managed so-called ‘protesters’ at the White House had little to do with the memory of George Floyd. They were just there to cause trouble. The @SecretService handled them easily.” Then he seemed to encourage his own supporters to come to Pennsylvania Avenue: “Tonight, I understand, is MAGA NIGHT AT THE WHITE HOUSE???”

Bowser tweeted back, “I call upon our city and our nation to exercise great restraint even while this President continues to try to divide us. Our power is in peace, in our voices and ultimately at the ballot box in November.”

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The Washington Post’s Peter Hermann, Michelle Boorstein, Joe Heim, Lauren Lumpkin, Perry Stein, Laura Vozzella, Hannah Natanson, Clarence Williams and Julie Zauzmer contributed to this report.