Some showed up because they said they were angry; others, because they said they had not been angrier sooner. Maggie Mason, a new mother, said that for two weeks she could not go on Facebook because of news stories about children in detention centers, such as the audio published by ProPublica of immigrant children crying after being separated from their parents. Now, with her 7-week-old baby sleeping in the stroller next to her, she said it was time to come out.

Over the past month, marches across the country have cropped up, adding to the pressure on the Trump administration to yield to calls to end the practice of splitting up or detaining families.

“The idea of kids in cages and asylum seekers in prisons and moms being separated from breast-feeding children, this is just beyond politics, it really is just about right and wrong,” said Rep. Pramila Jayapal, a Democrat from Washington State. On Thursday, she was arrested with more than 500 other women who occupied a Senate office building as part of a Women’s March protest against Mr. Trump’s immigration policy.

Ms. Jayapal said she has visited a federal prison just south of Seattle and met with 174 women and several dozen men who had been transferred from the Texas border. She said she was moved by the stories of asylum seekers and parents — stories of family members killed, of children left behind, of violent physical attacks and domestic abuse.

“I promised them that I would get their stories out and I promised them I would do everything I could to reunite their families,” Ms. Jayapal said.