(CNN) Demonstrators in Mexico spurred US Customs and Border Protection to shut down an El Paso, Texas, border crossing, one of the country's busiest ports of entry, early Monday, according to agency spokesman Roger Maier.

Border officials shut down the crossing just before 2 a.m. (4 a.m. ET) after "a large and unruly group formed on the Mexican side," Maier said.

The demonstrators "posed a threat to overrun the facility," he said, forcing CBP to employ "port hardening measures" and preventing travelers from entering the United States via the Paso del Norte International Bridge.

No arrests were reported on the US side, Maier said.

Authorities began permitting pedestrians to cross a few hours after the shutdown, and by 8 a.m. (10 a.m. ET), they opened four inspection booths and traffic was moving slowly, CBP said.

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