SACRAMENTO (CBS SF) – Gov.-Elect Gavin Newsom has condemned the use of tear gas by U.S. agents on migrants attempting to cross the U.S.-Mexico border near San Diego over the weekend.

“That’s not my America. We’re a land of refuge. Of hope. Of freedom,” Newsom said on Twitter following Sunday’s crackdown. “And we will not stand for this.”

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Oakland Mayor Libby Schaaf, who has faced scrutiny by the Trump administration over her community’s sanctuary status, also criticized the action.

“Firing tear gas at those seeking asylum into our country is an unconscionable act of violence by our federal government,” the mayor said in a tweet early Monday.

Meanwhile, President Donald Trump on Monday defended the border crackdown, saying on social media that he would take the unprecedented step of closing the border permanently “if need be.”

Mexico looked set to shore up security near its border with the United States on Monday, and local authorities said that 39 migrants were arrested after a peaceful march devolved into mayhem when U.S. agents fired tear gas into Mexico to stop some migrants who tried to breach the international line.

The situation at the border devolved after a large group marched to the U.S. border to appeal for the U.S. to speed processing of asylum claims for Central American migrants marooned in Tijuana.

There, some attempted to get through the fencing and wire separating the two countries, leading U.S. agents to fire tear gas.

Central American migrants -mostly Hondurans- run along the Tijuana River near the El Chaparral border crossing in Tijuana, Baja California State, Mexico, near US-Mexico border, after the US border patrol threw tear gas from the distance to disperse them after an alleged verbal dispute, on November 25, 2018. (GUILLERMO ARIAS/AFP/Getty Images)

Central American migrants -mostly Hondurans- run along the Tijuana River near the El Chaparral border crossing in Tijuana, Baja California State, Mexico, near US-Mexico border, after the US border patrol threw tear gas from the distance to disperse them after an alleged verbal dispute, on November 25, 2018. (GUILLERMO ARIAS/AFP/Getty Images)

American authorities also shut down the nation’s busiest border crossing at San Ysidro for several hours at the end of the Thanksgiving weekend.

Lurbin Sarmiento, 26, of Copan, Honduras, said she had been with her 4-year-old daughter at a concrete riverbed, which had a trickle of water from the Tijuana River, when U.S. agents fired the gas.

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“We ran, but the smoke always reached us and my daughter was choking,” Sarmiento told The Associated Press, visibly shaken.

She said she never would have gotten that close with her daughter if she thought there would be tear gas.

Fumes were carried by the wind toward people who were hundreds of feet away.

“We ran, but when you run the gas asphyxiates you more,” said Honduran migrant Ana Zuniga, while cradling her 3-year-old daughter Valery in her arms.

As the chaos unfolded, shoppers just yards away on the U.S. side streamed in and out of an outlet mall, which eventually closed.

Throughout the day, U.S. Customs and Border Protection helicopters flew overhead, while U.S. agents on foot watched beyond the wire fence in California.

Department of Homeland Security Secretary Kirstjen Nielsen said in a statement that U.S. authorities will continue to have a “robust” presence along the Southwest border and that they will prosecute anyone who damages federal property or violates U.S. sovereignty.

“DHS will not tolerate this type of lawlessness and will not hesitate to shut down ports of entry for security and public safety reasons,” she said.

More than 5,000 migrants have been camped in and around a sports complex in Tijuana after making their way through Mexico in recent weeks via caravan. Many hope to apply for asylum in the U.S., but agents at the San Ysidro entry point are processing fewer than 100 asylum petitions a day.

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© Copyright 2018 CBS Broadcasting Inc. All Rights Reserved. The Associated Press contributed to this report.