For decades, those who could reasonably argue they were fleeing persecution in their homelands could enter the United States and wait for their hearings in court. Often they stayed with relatives in the interior of the country.

But under a Trump administration program introduced in January, many migrants seeking admission to the United States are now being sent back to Mexico for the duration of their court proceedings. They are allowed to cross the border only for their hearings, often conducted in an expanding network of tent courts.

The migrants’ chances of gaining entry into the United States were further impaired last month when the Supreme Court let stand another new Trump administration policy that requires migrants who traveled through other countries en route to the United States to prove they had been denied asylum along the way before being eligible to apply at the border.

More than 50,000 migrants have been sent back from various southern border entry points, including about 120 a day who are being returned to Matamoros.

Not only has the plaza encampment near the bridge in Matamoros grown more crowded by the day, say volunteers who have worked with the group, but in a town that is notorious for drug and gang violence, some migrants have been victims of kidnapping for extortion and sexual assault.

Thursday’s protest at the bridge was peaceful, witnesses said. By early afternoon about 150 migrants remained, despite reminders by the authorities that the protocols for admission would not be waived. After a visit by immigrant advocates and the mayor of Matamoros, they dispersed in the late afternoon and the lanes reopened to traffic.

The Gateway bridge closure had caused long lines to form at two other international bridges leading into Texas, and border officials said Thursday’s scheduled immigration court hearings in Brownsville were postponed.