TIJUANA, Mexico — A peaceful march by Central American migrants waiting at the southwestern US border veered out of control on Sunday afternoon, as hundreds of people tried to evade a Mexican police blockade and run toward a giant border crossing that leads into San Diego.
In response, the US Customs and Border Protection agency shut the border crossing in both directions and fired tear gas to push back migrants from the border fence. The border was reopened later Sunday evening.
Soon after the migrants began their midmorning march to the border in Tijuana, they were met by Mexican federal police officers at a bridge that leads to the San Ysidro border crossing, through which millions of people pass each year. At that point, many of the marchers bypassed the police by running across a dry riverbed.
Police, carrying anti-riot shields, formed a new line and appeared to contain the rush of migrants 100 yards or more from the crossing. They erected metal barriers on the roads and sidewalks leading to the main border crossing for cars and trucks.
A smaller group of migrants then tried to reach a train border crossing, where they were stopped by tear gas released by US border officers.
After the gas had cleared, Mexican federal police officers pushed back protesters from the area of the train crossing.
Customs and Border Protection officers also released tear gas at a separate point a few hundred yards away to drive back the migrants.
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 toward people who were hundreds of feet away.
‘‘We ran, but when you run the gas asphyxiates you more,’’ Zuniga told the Associated Press while cradling her 3-year-old daughter Valery.
Some of the migrants told The New York Times they thought they could negotiate with US officials, but as they approached the metal border fence topped with barbed wire, they were met with several rounds of tear gas. A few men tried to climb the wall but fell back in the face of the gas.
At least two dozen tear gas canisters could be seen on the Mexican side of the border after the migrants turned back.
On Twitter, Democratic Senator Brian Schatz of Hawaii wrote that the “tear gas across the border against unarmed families is a new low.”
Kirstjen Nielsen, the Homeland Security secretary, said in a statement that some migrants trying to breach the fence had thrown projectiles at Customs and Border protection workers.
Mexico’s Interior Ministry said that 500 people were involved in the march Sunday, a fraction of the total number in the caravan, and that those who had attacked federal police would be deported. Municipal officials said 39 people had been arrested.
The unrest in Tijuana comes amid broader discussions about how to deal with the growing number of migrants fleeing poverty and violence in Central America who are amassed at crossing points in Tijuana and elsewhere along the border.
The backlog of people waiting to request asylum at a checkpoint has swelled, causing frustration among the migrants to boil over. Some of those rushing the border on Sunday had children in strollers and in their arms.
“The longer the caravaners stay in Tijuana, the more likely they are to succumb to the temptation to cross illegally into the US,” said Wayne Cornelius, professor emeritus of political science at the University of California San Diego and an expert on the border.
Mexico inaugurates a new president, Andrés Manuel López Obrador, Saturday, and top Cabinet officials in his incoming government had planned to meet Sunday to discuss possible solutions for dealing with the population, including potentially allowing migrants applying for asylum in the United States to stay in Mexico while they are being processed.
The Trump administration has demanded that Mexico agree to such a shift in policy. Currently, asylum seekers are allowed to stay in the United States as they wait for their hearing before an immigration judge. The wait can last years, during which time many migrants are allowed to work.
But the meeting of the incoming administration officials in Mexico City was derailed by the chaotic events along the San Ysidro border crossing. The focus of the meeting immediately shifted to the day’s crisis and the political implications it might have.
Trump wrote on Twitter on Sunday that it “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.”
Thousands of migrants began arriving in Tijuana about 10 days ago and have been housed since then in squalid conditions in a community sports center that has been converted into a makeshift shelter. Many have become desperate with the realization of the obstacles still before them in reaching the United States.
Tijuana city officials say they have no money to improve conditions at the sports center, where more than 5,000 migrants are sheltering in a space with capacity for no more than 3,500.
Fani Caballero, 32, a migrant from Honduras, sat by the train tracks, within sight of US agents. Her daughter, Cristina, 7, cried as Customs and Border Protection helicopters circled overhead.
“People had thought that they were going to open the gates, but that was a lie,” Caballero said. “We thought it would be easier.”