As protests quickly flipped from peaceful to fiery in more than two dozen U.S. cities, President Trump said little Saturday about the frustrations that drove thousands of people to crowd into downtown streets in the middle of a pandemic. Instead, the president defaulted to his usual style of leadership: tearing people down and talking tough.

“Mayor Jacob Frey of Minneapolis will never be mistaken for the late, great General Douglas McArthur [sic] or great fighter General George Patton,” Trump tweeted Saturday of the Democrat whose city was in flames. “How come all of these places that defend so poorly are run by Liberal Democrats? Get tough and fight (and arrest the bad ones). STRENGTH!”

Trump blasted demonstrators who had confronted Secret Service agents outside the White House as “professionally managed so-called ‘protesters’ ” who “were just there to cause trouble.” And he seemed to savor a confrontation, tweeting that “Tonight, I understand, is MAGA NIGHT AT THE WHITE HOUSE???”

There was no talk of uniting the country and, hours later, only brief mention of those protesting racial injustice, police brutality and the killing by a white police officer of a black man, George Floyd, in Minneapolis. Even then, Trump maintained the strategy he has used throughout his tenure, emphasizing the nation’s divisions and seeking to capitalize on them.

“We support the right of peaceful protests and we hear their pleas, but what we are now seeing on the streets of our cities has nothing to do with the memory of George Floyd,” Trump said after watching the SpaceX launch in Florida. “The violence and vandalism is being led by antifa and other radical left-wing groups who are terrorizing the innocent, destroying jobs, hurting businesses and burning down buildings.”

In the absence of presidential leadership, and with crowds gathering yet again Saturday night, members of Congress, governors and mayors need to step up, said Princeton University historian Julian Zelizer.

“If some leaders don’t offer a prospect of change in the next few days, that sense of hopelessness we’re seeing on the streets just gets worse,” said Zelizer, who studies the 1960s and ’70s, a period of similar social and economic upheaval when divisions over race and class exploded into riots.

“Good leaders cannot separate themselves from the turbulence,” Zelizer said. “If they’re silent, or if they’re too distant, it just adds to the frustration people feel.”

There is no magic formula for averting a long, hot summer of street violence and social discord, of raging flames and physical confrontations. Through several nights of intensifying protests following Floyd’s death on Monday, few moments have offered any promise of a healing connection. Rather, the people Americans have chosen to take charge in times of crisis have more often left a leadership vacuum — such as the remarkable absence of police and public officials on the streets of Minneapolis in recent days.

A rare exception came on Friday night in Atlanta. As fires burned and angry crowds banged up against lines of police officers, the city’s police chief, Erika Shields, waded into a clot of protesters and listened to their grievances.

Shields told the protesters that bad cops must be “weeded out and fired before they can choke somebody.” She told them again and again, “I’m with you.” She gently touched their arms.

“I’m standing here because what I saw was my people face to face with this crowd, and everybody’s thinking ‘How can we use force to defuse it?’ And I’m not having it. I’m not having it,” Shields told the crowd. “You have a right to be upset and be scared and to want to yell.”

Demonstrators thanked the chief, held her hand and cried to her about their fear for their children’s lives.

By contrast, the lack of evident leadership on the streets of Minneapolis on Thursday and Friday nights — a source of wonderment among TV commentators across the ideological spectrum — led to the sense that no one was in charge. Businesses burned to the ground. The nation watched on live TV as crowds looted liquor stores and other shops, with no pushback from the authorities.

Minnesota officials seemed to speak of everything but a constructive path forward. Rep. Ilhan Omar (D), who represents Minneapolis, posted a video saying that “we can’t ask our community to be peaceful if we continue to not deliver justice for them.”

Minnesota Gov. Tim Walz (D) denounced the protests, calling Friday’s violence “a mockery of pretending this is about George Floyd’s death or inequities or historical traumas to our communities of color.”

The sight of demonstrators breaching and setting fire to the Minneapolis police department’s 3rd Precinct station on Thursday became a symbol of the government’s loss of control. Frey, the city’s first-term mayor, said he ordered police to abandon the station.

“The symbolism of a building cannot outweigh the importance of life, of our officers or to the public,” he said.

But Lucy Gerold, who spent 31 years with the Minneapolis police, the last six as commander of the 3rd Precinct, called that decision “an inexperienced and naive response” that sent the wrong message to demonstrators.

City leaders “just said okay, if we just give them the precinct, we’ll feed the beast and they’ll be satisfied,” said Gerold, who left the department in 2014. “That’s just feeding the fire. . . . I think that opened the door for the rest of it.”

Probably no political gesture, speech or legislative action could have prevented this week’s explosion of frustrations. But in the long, ugly history of American political street violence, the enduring images of healing often involve dramatic scenes of politicians and police who, rather than facing off against protesters, waded in for tough, painful confrontations that pointed a path toward reforms.

On the night the Rev. Martin Luther King Jr., was assassinated in 1968, as 34 U.S. cities burned in riots of grief and anger, Sen. Robert F. Kennedy stepped before a crowd in an Indianapolis neighborhood already engulfed in protest. Police had declined to accompany Kennedy because the crowd was too hot.

Standing on a flatbed truck, Kennedy said to a virtually all-black crowd: “You can be filled with bitterness, with hatred, and a desire for revenge. We can move in that direction as a country, in great polarization — black people among black, white people among white, filled with hatred toward one another.

“Or we can make an effort, as Martin Luther King did, to understand and to comprehend, and to replace that violence, that stain of bloodshed that has spread across our land, with an effort to understand with compassion and love.”

Kennedy spoke that night for the first time in public about the killing of his brother, the president. “For those of you who are black and are tempted to be filled with hatred and distrust at the injustice of such an act, against all white people, I can only say that I feel in my own heart the same kind of feeling. I had a member of my family killed, but he was killed by a white man.”

The president at that moment, Lyndon Johnson, made no such gesture as U.S. cities burned. He felt betrayed, believing that by winning historic civil rights laws, he had done more for the country’s black population than any president in decades.

Trump, too, has perceived the week’s violence through the prism of his own political status. Retweeting a list of top Minnesota politicians, all Democrats, he wrote, “Time for a change!”

On Saturday afternoon, Trump urged the nation’s mayors to “get tougher,” and he dismissed protesters as “a lot of radical left bad people.”

D.C. Mayor Muriel E. Bowser (D), whom the president had accused of failing to provide police support for Secret Service officers guarding the White House, blasted Trump on Saturday for his brand of leadership.

“We need leaders who . . . in times of great turmoil and despair can provide us a sense of calm and a sense of hope. Instead, what we’ve got in the last two days from the White House is the glorification of violence against American citizens,” Bowser said.

The lack of a clear, consistent message helped create a sense of anarchy that fed reactions ranging from disappointment to denunciation around the world. America’s rivals and adversaries jumped on the violence as proof that the United States treats its people unjustly.

Turkey’s leader, Recep Tayyip Erdogan, said that a “racist and fascist approach” led to Floyd’s death and was a symbol of an “unjust order” in the United States. In Iran, people held a candlelight vigil in memory of Floyd. In China, state television broadcast a commentary contending that excessive police force “shows the deep social contradictions” in the United States.

Trump continued to talk tough; on Saturday, he reiterated an offer to make the Army available to suppress riots, saying, “We have our military ready, willing and able.”

Other political leaders also made clear that the street violence was unacceptable. Atlanta’s mayor directed a stirring, tough message to protesters who were setting fires on the city’s streets.

“This is not a protest,” said Mayor Keisha Lance Bottoms (D). “This is not in the spirit of Martin Luther King Jr. This is chaos. A protest has purpose. . . . You’re not protesting anything running out with brown liquor in your hands and breaking windows in this city. . . . Go home!”

But there was little talk about the kinds of reforms that might at last repair relations between police and black Americans.

“That’s not going to come from this president, but policing is still mostly a local and state issue,” Zelizer said. “Just as we’ve seen with the pandemic, governors can and will step up.”

The reports from blue-ribbon commission and congressional investigations produced following past rounds of urban street violence would test the strength of any bookshelf. And that process has brought changes that have reduced the use of force in many police departments, especially in big cities.

But police brutality and racial bias continue to hold explosive power as political issues, and few elected officials spend much energy confronting the problem. It’s easier for some leaders to traffic in pat slogans, bashing either the police or the protesters.

Trump’s use of the phrase “when the looting starts, the shooting starts” — coined by a former Miami police chief and used at campaign rallies by the 1968 independent presidential candidate, segregationist George Wallace — was a reminder that it can be all too easy to use division and discord to bolster a political following.

Wallace built a movement with overtly racist appeals in 1968, lashing out at protesters. “If any demonstrator lies down in front of my car when I’m president, that’ll be the last car he lays down in front of,” he said.

Richard Nixon, the Republican candidate for president that year, blasted antiwar protesters. “They call themselves flower children. I call them spoiled rotten,” he said. But as the nation suffered through a summer of street violence, Nixon softened his campaign, settling for a while on a more hopeful message.

In one campaign TV ad, he summoned “the voice of the great majority of Americans, the forgotten Americans, the non-shouters, the non-demonstrators. . . . They are black, and they are white; they’re native-born and foreign-born; they’re young, and they’re old. . . . They know that this country will not be a good place for any of us to live in unless it is a good place for all of us to live in.”

In 2016, Trump adopted not Nixon’s approach, but Wallace’s message, the one that clicked with disaffected, frustrated white voters — a politics of grievance, featuring full-throated attacks on elites, government and the media.

Late Saturday, as the nation braced for another scary night, Trump continued his campaign, tweeting, as ever: “Fake News is the Enemy of the People!”

Mark Berman contributed to this report.