WASHINGTON -- They wore white. They shook their fists in the air. They carried signs reading: "No more children in cages," and "What's next? Concentration Camps?" In major cities and tiny towns, hundreds of thousands of marchers gathered Saturday across America, moved by accounts of children separated from their parents at the U.S.-Mexico border, in the latest act of mass resistance against President Trump's immigration policies.

Protesters flooded more than 700 marches, from immigrant-friendly cities like New York and Los Angeles to conservative Appalachia and Wyoming. They gathered on the front lawn of a Border Patrol station in McAllen, Texas, near a detention center where migrant children were being held in cages, and on a street corner near Mr. Trump's golf resort at Bedminster, New Jersey, where the president is spending the weekend.

Mr. Trump has backed away from family separations amid bipartisan and international uproar. His "zero tolerance policy" led officials to take more than 2,000 children from their parents as they tried to enter the country illegally, most of them fleeing violence, persecution or economic collapse in their home countries.

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"You have people fleeing persecution and violence, put into military bases as if they're Osama Bin Laden or something," Faiz Shakir, the national political director at ACLU, told CBS News.

Those marching Saturday demanded the government quickly reunite the families that were already divided.

A Brazilian mother separated from her 10-year-old son more than a month ago approached the microphone at the Boston rally.

"We came to the United States seeking help, and we never imagined that this could happen. So I beg everyone, please release these children, give my son back to me," she said through an interpreter, weeping.

"Please fight and continue fighting, because we will win," she said. The crowd erupted.

In Washington, D.C., an estimated 30,000 marchers gathered in Lafayette Park across from the White House in what was expected to be the largest protest of the day, stretching for hours under a searing sun. Firefighters at one point misted the crowd to help people cool off.

Lin-Manuel Miranda, creator of the musical "Hamilton," sang a lullaby dedicated to parents unable to sing to their children. Singer-songwriter Alicia Keys read a letter written by a woman whose child had been taken away from her at the border.

"It's upsetting. Families being separated, children in cages," said Emilia Ramos, a cleaner in the district, fighting tears at the rally. "Seeing everyone together for this cause, it's emotional."

Around her, thousands waved signs: "I care," some read, referencing a jacket that first lady Melania Trump wore when traveling to visit child migrants. The back of her jacket said, "I really don't care, do U?" and it became a rallying cry for protesters Saturday.

"I care!! Do you?" read Joan Culwell's T-shirt as she joined a rally in Denver.

"We care!" marchers shouted outside Dallas City Hall. Organizer Michelle Wentz says opposition to the Trump administration's "barbaric and inhumane" policy has seemed to transcend political lines.

"This is the issue crossing the line for a lot of people," said Robin Jackson, 51, of Los Angeles, who protested with thousands carrying flags, signs and babies.

Singer John Legend serenaded the crowd and Democratic politicians who have clashed with Mr. Trump had strong words for the president, including U.S. Rep. Maxine Waters who called for his impeachment.

The president took to Twitter amid the protests, first to show his support for Immigration and Customs Enforcement as some Democrats called for major changes to the agency. Tweeting Saturday from New Jersey, Mr. Trump urged ICE agents to "not worry or lose your spirit" and wrote that "the radical left Dems want you out. Next it will be all police."

He later tweeted that he never pushed House Republicans to vote for immigration overhaul measures that failed last week, contradicting a post three days ago in which he urged GOP congressional members to pass them.

In Mr. Trump's hometown of New York City, another massive crowd poured across the Brooklyn Bridge in sweltering 90-degree heat, some carrying their children on their shoulders, chanting, "Shame!" Drivers honked their horns in support.

"It's important for this administration to know that these policies that rip apart families -- that treat people as less than human, like they're vermin -- are not the way of God, they are not the law of love," said the Rev. Julie Hoplamazian, an Episcopal priest marching in Brooklyn.

Though seasoned anti-Trump demonstrators packed the rallies, others were new to activism, including parents who said they felt compelled to act after heart-wrenching accounts of families who were torn apart.

Marchers took to city parks and downtown squares from Maine to Florida to Oregon; in Alaska, Hawaii and Puerto Rico; on the international bridge between El Paso, Texas, and Juarez, Mexico; even in Antler, North Dakota, population 27. Some of the demonstrations were boisterous, others were quiet.

Five people were arrested outside an ICE office in Dallas for blocking a road. At least one arrest was made in Columbus, Ohio, when protesters obstructed a downtown street. Light-rail service temporarily shut down in Minneapolis as thousands of demonstrators got in the way of the tracks. A rally in Portland, Maine, grew so large that police had to shut down part of a major street.

But in Dodge City, Kansas, a 100-person rally led by a Catholic church felt more like a mass than a protest.

In rural Marshalltown, Iowa, about 125 people gathered for a march organized by Steve Adelmund, a father of two who was inspired after turning on the news on Father's Day and seeing children being separated from their families and held in cages.

"It hit me in the heart. I cried," he said.

"If we can't come together under the idea of 'Kids shouldn't be taken from their parents,' where are we?" he asked. "We have to speak out now while we can, before we can't."

Drum beats and horns sounded as thousands of protesters took to the streets of San Francisco.

"We came here to let the president know that this is not acceptable," said San Francisco resident Barry Hooper, who attended with his wife and two daughters.

His 7-year-old daughter Liliana clutched a sign she made, saying, "Stop the separation."

Three thousand miles away in Washington, protesters ended their march at the white-columned Justice Department. They stacked their protest signs, written in English and Spanish, against its grand wooden doors.

"Fight for families," one sign demanded.ght-wing activists with the group Patriot Prayer also have a permit to march later in the day Saturday and the Portland Police Bureau said Friday they planned to have a heavy police presence.

Sharaf and co-organizer Erin Conroy have coordinated with immigrant advocacy groups.

"This is not my wheelhouse," Conroy said. "As far as I'm concerned, this is a national emergency that we all need to be focused on right now."

Immigration attorney Linda Rivas said groups have met with U.S. authorities, congressional representatives and other leaders to discuss an escalating immigration crackdown that they say began decades ago. But the family separation policy has been a watershed for attracting a broader spectrum of demonstrators, she said.

"To finally have people on board wanting to take action, marching, taking to the streets," Rivas said. "It's been motivating for us as advocates because we have to keep going."

On Thursday, more than a thousand people and organizations including the ACLU, the ACLU of Texas and United We Dream gathered in Texas to demand that separated families be immediately reunited, the ACLU said in a news release. They gathered in Brownsville, which according to the ACLU is one of the "hardest hit" areas under the Trump administration's "zero tolerance" policy.

"The administration doesn't have a plan to reunite families. But we have a plan," said Terri Burke, executive director of the ACLU of Texas. "We are going to keep fighting. Refugees are welcome here, immigrants are welcome here. We won't stop until every single child is reunited."