Hearings in tent courts in South Texas are underway for asylum-seekers forced to wait in Mexico while their immigration applications are considered.
Monday's hearings mark the formal opening of the courts. Twenty-one migrants lined up in Mexico to cross the border for their appearances.
Officials say they want 200 migrants appear each day in the tents in Laredo.
A judge in a brick courthouse in San Antonio is presiding over the Laredo court via video chat.
Outside observers are barred from the tents, but journalists were allowed into the San Antonio courthouse.
The Trump administration introduced its "Remain in Mexico" policy in January in response to an increase in asylum-seeking families, especially from Central America.
Critics have assailed the policy for making families and children wait in violent Mexico border cities.