Migrants approaching the US border from Mexico have been enveloped with tear gas after a few tried to breach the fence separating the two countries.
US agents shot the gas and children were seen screaming and coughing in the mayhem.
Honduran migrant Ana Zuniga, 23, said she saw migrants open 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.
"We ran, but when you run the gas asphyxiates you more," she told the AP while cradling her 3-year-old daughter Valery in her arms.
Mexico's Milenio TV also showed images of several migrants at the border trying to jump over the fence.
US Border Patrol helicopters flew overhead, while agents held vigil on foot beyond the wire fence in California.
The Border Patrol office in San Diego said via Twitter that pedestrian crossings have been suspended at the San Ysidro port of entry at both the East and West facilities. All northbound and southbound traffic was halted.
Earlier on Sunday, some Central American migrants pushed past a blockade of Mexican police standing guard near the international border crossing.
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 US, but agents at the San Ysidro entry point are processing fewer than 100 asylum petitions a day.
Some of the migrants who went forward Sunday called on each other to remain peaceful.
They appeared to easily pass through the Mexican police blockade without using violence.
A second line of Mexican police carrying plastic riot shields stood guard outside a Mexican customs and immigration plaza, where the migrants were headed.
That line of police installed tall steel panels behind them outside the Chaparral crossing on the Mexican side of the border, which completely blocked incoming traffic lanes to Mexico.
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.
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.
Additional reporting by Associated Press.
