What to Know
More than 5,000 migrants have been camped in and around a sports complex in Tijuana.
Roughly 5,800 troops were deployed to the area as part of "border hardening" efforts by the U.S.
The San Ysidro Port of Entry is the border's busiest crossing, with about 110,000 people entering the U.S. every day.
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 border crossings 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.
They carried hand-painted American and Honduran flags while chanting: "We are not criminals! We are international workers!"
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. Mexico's 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.
Mexico's Interior Ministry said around 500 migrants tried to "violently" enter the U.S.
The ministry said in a statement it would immediately deport those people and would reinforce security.
Tijuana Police arrested 39 people and turned them over to Mexico's National Institute of Migration, according to the agency's Facebook page. Migrants will be deported to their countries of origin for inciting violence and disorderly conduct, the post said.
As the chaos unfolded, shoppers just yards away on the U.S. side streamed in and out of an outlet mall.
Throughout the day, U.S. Customs and Border Protection helicopters flew overhead, while U.S. agents held vigil on foot beyond the wire fence in California.
On Sunday afternoon, the Border Patrol office in San Diego said via Twitter that pedestrian crossings were reopened at the San Ysidro port of entry at both the East and West facilities for all northbound and southbound traffic.
The pedestrian crossings had been suspended earlier in the day. Interstates 805 and 5 and state Route 905, which were partially shut down, reopened shortly after 5 p.m.
The San Ysidro Port of Entry is the busiest land border crossing in the world, with about 110,000 people entering the U.S. every day. That traffic includes some 40,000 vehicles, 34,000 pedestrians and 150 to 200 buses.
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.
Irineo Mujica, who has accompanied the migrants for weeks as part of the aid group Pueblo Sin Fronteras, said the aim of Sunday's march toward the U.S. border was to make the migrants' plight more visible to the governments of Mexico and the U.S.
"We can't have all these people here," Mujica told The Associated Press.
Nearly 20 migrant rights and social justice groups make up the San Diego Migrant and Refugee Solidarity Coalition, which hosted a march on the U.S. side. The coalition and others started at Larsen Field on Camino de la Plaza. The group said President Trump created a “war-like situation at the border.”
Other marches were planned across the nation, including in L.A., Sacramento, and San Francisco.
Tijuana Mayor Juan Manuel Gastelum on Friday declared a humanitarian crisis in his border city of 1.6 million, which he says is struggling to accommodate the crush of migrants.
U.S. President Donald Trump took to Twitter Sunday to express his displeasure with the caravans in Mexico.
"Would be very SMART if Mexico would stop the Caravans long before they get to our Southern Border, or if originating countries would not let them form (it is a way they get certain people out of their country and dump in U.S. No longer)," he wrote.
Mexico's Interior Ministry said Sunday the country has sent 11,000 Central Americans back to their countries of origin since Oct. 19, when the first caravan entered the country. It said that 1,906 of those who have returned were members of the recent caravans.
Mexico is on track to send a total of around 100,000 Central Americans back home by the end of this year.