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 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, 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 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.