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A group of mothers, carrying their children, on Tuesday denounced the Trump administration’s plan to separate families who enter the United States illegally, walking out of a hearing in Washington where the homeland security secretary was speaking.

About two dozen mothers affiliated with women's rights or immigrants' rights groups walked out in "silent protest" during a Senate subcommittee hearing where Homeland Security Secretary Kirstjen Nielsen was set to testify,Heidi Altman, director of policy at the National Immigrant Justice Center, told NBC News.

“We attended the hearing with our little ones, and then in order to protest the family separation policy walked out when Secretary Nielsen began speaking,” she said after the demonstration.

“This feels like it’s not normal business as usual. It’s a policy that’s really so cruel and so harmful in its immediate impact that it felt like it called for an unusual response,” she said.

On Monday, Attorney General Jeff Sessions announced that the Justice Department was seeking to prosecute everyone who crossed into the country illegally and would separate children from families, instead of keeping them in detention together.

"If you are smuggling a child then we will prosecute you, and that child will be separated from you as required by law," Sessions said at a law enforcement conference in Scottsdale, Arizona. "If you don't like that, then don't smuggle children over our border."

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That policy would apply to families seeking asylum who do not surrender to immigration authorities at a port of entry, a move decried by the advocates. Children of adults who try to seek asylum at a designated port of entry will not be separated from their parents, administration officials said.

“Our government, the United States of America, is separating families who are coming to our borders seeking protection, and when I say that I mean literally ripping children out of their parents' arms because they’ve come here asking for help,” Michelle Brané, director of the migrant rights and justice program at the Women’s Refugee Commission, said in remarks before the hearing.

“To be punishing these parents for wanting to do that and punishing these children who have no say in the matter is outrageous to me,” she told NBC News Tuesday afternoon.

The Department of Homeland Security says 700 immigrant children have been separated from their parents since the fiscal year began last October. Officials said adults have been separated from children in the past if border officers could not be certain that they were actually the parents of the children accompanying them or if the adults were prosecuted for a crime.

Thomas Homan, deputy director of U.S. Immigration and Customs Enforcement, said Monday that the Department of Homeland Security "does not have a blanket policy on separating families as a deterrent."

"There is no new policy. This has always been the policy. But you will see more prosecutions because of the commitment to zero tolerance" of illegal border entries," he said.

Jen Podkul, director of policy with Kids in Need of Defense, said separating children from their families would make proving asylum cases more difficult, since parents or caregivers often have documents or other information attorneys need.

“Our job has just gotten a lot harder,” she said in remarks before the hearing. The group provides legal defense for unaccompanied children.

Podkul said the policy would also exacerbate the backlog of immigration cases judges face.

“It’s not just the cruelty of it, but the absurdity, the inefficiency of this,” she said.

Podkul told NBC News that she hoped that the demonstration sent a message of solidarity with mothers whose children have been taken from them.

“These mothers can’t speak for themselves — you can’t see them walking through the halls of Congress,” she said.