Migrants who want to request asylum have camped out on the international bridge leading into Brownsville, Texas, causing a closure of the span.
Dozens of migrants from Central America and elsewhere stretched out on the bridge before dawn, with some sleeping.
A Mexican official who was not authorized to be named confirmed the blockage of the bridge leading out of Matamoros, Mexico.
The official said the migrants were tired of waiting to make their initial claims for asylum at a U.S. border crossing.
Under a policy know as metering, U.S. officials at many border bridges accept only a few asylum-seekers per day. The Associated Press found about 19,000 names on waiting lists in four border cities visited in late July.