TIJUANA, Mexico (AP) U.S. border agents fired tear gas on hundreds of migrants protesting near the border with Mexico on Sunday after some of them attempted to get through the fencing and wire separating the two countries, and American authorities shut down the nations busiest border crossing from the city where thousands are waiting to apply for asylum.

The situation devolved after the group began a peaceful march to appeal for the U.S. to speed 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 below the bridge. More police carrying plastic riot shields were on the other side, but migrants walked along the river to an area where only an earthen levee and concertina wire separated them from U.S. Border Patrol agents.

Some saw an opportunity to breach the crossing.

An Associated Press reporter saw U.S. agents shoot several rounds of tear gas after some migrants attempted to penetrate several points along the border. Mexicos Milenio TV showed images of migrants climbing over fences and peeling back metal sheeting to enter.

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

Children screamed and coughed. 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, Zuniga told the AP while cradling her 3-year-old daughter Valery in her arms.