"Stop this inhumane, barbaric policy, rescind your actions, take responsibility for it," Pelosi said at a midday news conference.
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She was joined at the podium by Rep. Juan Vargas, Rep. Judy Chu, Rep. Lou Correa, Rep. Michelle Lujan Grisham and others. Pelosi and the other leaders toured four detention facilities near the border.
MORE: Hundreds of children wait in Border Patrol facility in Texas
McAllen detention center
Pelosi said Republicans have a moral and legal responsibility to end a policy that has led to children being separated from their families along the border between the U.S. and Mexico.
She added that ripping vulnerable children away from their parents is "an utter atrocity that debases America's values and our legacy as a beacon of hope, opportunity and freedom."
"This is not about immigration. This is about humanity. It's about family. It's about who we are as a country," Pelosi said.
President Donald Trump has sought to blame Democrats for the family separations, but Pelosi said the "blame for every mistreated child, heartbroken mother and father and broken family rests squarely on the president, and only he can end the trauma."
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MORE: Everything you need to know about the immigrant family separation controversy
Everything you need to know about the immigrant family separation controversy
Chu echoed Pelosi's opinions, saying the president can end the separation of children from their parents with a simple phone call.
"It's up to President Trump to own the fact that he made this policy, that he made this zero-tolerance policy. Trump started this and Trump can end this. He can pick up the phone and he can stop this," Chu said.
Currently, thousands are in custody, and many are innocent children who are caught up in the fight over undocumented immigrants.
The Trump administration's "zero tolerance" policy of arresting undocumented immigrants and asylum seekers and separating them from their children for months is adding fuel to the fire.
MORE: Former first lady Laura Bush calls zero-tolerance immigration policy 'cruel' and 'immoral'
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Laura Bush statement on immigration policy
Images and video released by the Department of Homeland Security show fenced-in cells where men, women and children are being held temporarily, some on bare floors.
As enforcement is beefed up along the border, some officials claim there is no policy to separate families, despite 2,000 children already being taken.
Pelosi said that number of children is expected to increase to 30,000 by August.
Trump continues blaming Democrats for children being separated from their families even though this is his policy. Meantime, some critics say Republicans are holding the kids hostage, using them as bargaining chips. The policy is not law and could be stopped by the administration without any help from Democrats.
The Associated Press and ABC News contributed to this report.