Hundreds of people chanted "families belong together" in front of the US Justice Department on Friday, accusing the White House of violating human rights and traumatizing children for political reasons.
US administration has a plan to separate asylum-seeking Central American immigrant children from their parents.
The protests came after the Trump administration confirmed that it had split hundreds of families who crossed the US-Mexican border without immigration documents since October.
Each week, thousands cross the US-Mexico border and immediately turn themselves in to authorities asking for asylum.
In April alone, 50,924 people were detained after crossing the border without papers, including 4,314 unaccompanied children and 9,647 family units, according to US Customs and Border Patrol.
Last month, the head of the US Justice Department, Attorney General Jeff Sessions, announced an official policy of arresting and separating all parents from their children if they cross the border illegally.
The government sees the policy as a necessary deterrent to illegal immigration, but critics say it is cruel to refugees fleeing violence from Central America countries like El Salvador, Guatemala and Honduras.
The American Civil Liberties Union (ACLU) has filed a lawsuit challenging the policy, calling it a violation of human rights. "Separating families is more than cruel and unnecessary - it's torture," the ACLU said.
Trump has shown little tolerance for undocumented immigrants since taking office last year.
The Republican president has promised to build a wall on the border with Mexico to stop migrant flow into the US.
