Rev Julie Hoplamazian - an Episcopal priest marching in Brooklyn - said: "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."
It was the latest act of mass resistance against President Donald Trump's immigration policies.
There were 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 centre where migrant children were being held in cages, and on a street corner near Trump's golf resort at Bedminster, New Jersey, where the president is spending the weekend.
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.
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.
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 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.
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