MEXICO has pledged to shore up its security after United States agents fired tear gas at migrants who were attempting to cross the border.

Tijuana’s municipal government said more than three dozen migrants were arrested for disturbing the peace and other charges stemming from chaos that followed a peaceful demonstration at the border.

Mexico’s Interior Ministry added it would immediately deport those who tried to “violently” enter the US from Tijuana.

The situation devolved after the group began a peaceful march to appeal for the US to speed up processing of asylum claims for Central American migrants marooned in Tijuana.

Mexican police had kept them from walking over a bridge leading to the Mexican port of entry, but the migrants pushed past officers to walk across the Tijuana River.

Honduran Ana Zuniga, 23, said she saw migrants opening a small hole in concertina wire at a gap on the Mexican side of a levee, at which point US agents fired tear gas at them.

Children screamed and coughed. Fumes were carried by the wind towards people who were hundreds of feet away.

“We ran, but when you run the gas asphyxiates you more,” Zuniga said while cradling her three-year-old daughter Valery.

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

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