US authorities on Sunday temporarily shut the San Ysidro Port of Entry, one of the world’s busiest border crossings, after a group of hundreds of migrants from Central America evaded a phalanx of Mexican police, crossed a small river, and marched down a side street in front of a pedestrian crossing.

With helicopters circling overhead, US border authorities fired tear gas and rubber bullets as some of the marchers approached the United States at a metal fence where train tracks run through. At least one person crossed through the barrier and was detained by a US border agent, according to an eyewitness.

The marchers were close to the “El Chaparral” pedestrian border crossing shortly after going across a river that runs next to it. The group continued marching away from the border crossing. They then pushed in the direction of the vehicle border crossing and another pedestrian crossing in Tijuana a few minutes away. Mexican federal police had blocked the road where cars usually enter Mexico from the US with a large metal gate, and it seemed unlikely the migrants could have passed through it. Mexican federal police quickly cordoned off the area, making access impossible.