TIJUANA, Mexico — United States authorities closed off the busiest port of entry along the border with Mexico on Sunday and fired tear gas at members of a Central American migrant caravan who had rushed the fencing that separates the countries.

Although the number of people in the caravan was relatively small, the unrest, with migrants attempting to climb fences and run through car lanes to reach the U.S., and scenes of mothers and children choking on tear gas, represented a serious escalation of the caravan crisis.

What had begun Sunday morning as a migrant protest of the slow pace of the U.S. asylum-claims process devolved into a chaotic scramble in which hundreds made their way to the border, hoping to cross onto U.S. soil. To block that from happening, and as some threw rocks and bottles, U.S. authorities took the rare step of firing tear gas into Mexico as well as closing all legal vehicle and foot traffic to the San Ysidro border crossing that U.S. officials say normally has about 100,000 visitors per day.

Department of Homeland Security Secretary Kirstjen Nielsen said in a statement that the port of entry was closed “to ensure public safety in response to large numbers of migrants seeking to enter the U.S. illegally.”

Some of the migrants tried to breach the border fencing and “sought to harm CBP personnel by throwing projectiles at them,” the statement said, referring to Customs and Border Patrol.

Mexico’s Interior Ministry added that almost 500 migrants had tried to cross the border in a “violent manner.” Mexican authorities said they would deport anyone who tried to cross illegally.

President Trump has repeatedly threatened to close border crossings to prevent the migrant caravan from entering the U.S. Although members of the caravan have been in Tijuana for several days, this is the first time that a significant group has massed at the border fence.

The San Ysidro crossing is a large complex with multiple lanes of vehicle and pedestrian access points. In recent days, Mexican authorities and others have worried about the economic effect of the United States closing down such an important crossing for trade and travelers.

Duncan Wood, the director of the Mexico Institute at the Wilson Center in Washington, said in a statement that “the closing of the border is a drastic response to a serious incident but it is vital that calm heads prevail.”

“Closing a border crossing like (San Ysidro) more than temporarily involves losses of many millions of dollars and severe disruption of commerce and life at the border,” he added.

After starting in Honduras and traveling across Mexico, the first members of the caravan began arriving in Tijuana about two weeks ago. In recent days, the group has grown to more than 8,200, with about 7,400 members in the border cities of Tijuana and Mexicali, according to Mexican authorities. Tijuana’s mayor declared the situation a humanitarian crisis.

The bulk of the group has been camped out at a sports complex across a highway from the border fence. While a small portion of the caravan protested, thousands stayed behind in the sports complex. U.S. border officials have said they have a limited capacity to process asylum seekers, up to 100 per day. Asylum seekers waiting at the border said 40 per day were being let in on Saturday and Sunday. On Friday, 80 were allowed in.

For the past few days, the migrants planned Sunday’s protest to express their frustration about the wait and the conditions where they’re living, some of them painting signs asking Trump to let them in.

On Sunday morning, the protest march headed toward one of the pedestrian crossings. Mexican police in riot gear blocked its way, and a scuffle broke out between police and two dozen protesters. After being rebuffed, the situation grew more chaotic, with some migrants running across a dry canal and others trying to cross in different places.

U.S. authorities fired tear gas that wafted into Mexico and caused migrants to run from the fumes. No serious injuries were immediately reported.

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