Reuters video showed men, women and children waiting onto a bridge connecting Mexico and Guatemala, only to be met by dozens of Mexican police and immigration officials on the other side of the gate.
Many have vowed to stay put, demanding to be let through. But only a small number have trickled in.
Many of these migrant families are fleeing violence and poverty back home, with large swathes of Central America afflicted by gang violence and drug trafficking.
Earlier, Mexican Foreign Minister Luis Videgaray vowed to meet the “challenge” of the caravan after holding talks with U.S. Secretary of State Mike Pompeo in Mexico City.
Trump has threatened to shut down the U.S.-Mexico border if the caravan is not stopped, which could seriously disrupt trade.