All border crossings have been halted as hundreds of caravan migrants are trying to breach a Mexico-US border fence.
Several hundred Central American migrants stormed the international border crossing between Tijuana and California to pressure the US into hearing their asylum claims.
US agents began firing tear gas following clashes at the border after a small group of migrants kicked through part of a fence.
The migrants tried to protect themselves by covering their faces, with mothers holding their children close.
Most of the panicked migrants turned back and ran away amid the stinging gas as riot police began surrounding them.
Sky News' chief correspondent Stuart Ramsay said there were chaotic scenes as migrants got onto the train tracks which lead to the US, and there were "scuffles with the police" throughout.
"Some were able to get through a gap in the border fence - but on the other side the US border patrol were there and fired tear gas at them".
The migrants, mostly from Honduras, carried hand-painted American and Honduran flags, chanting: "We are not criminals! We are international workers!"
Some were seen scrambling up railway sidings in an attempt to reach the US border - with many young children at the centre of the mayhem.
Dozens of Mexican police with plastic riot shields stopped the migrants well ahead of the crossing on Sunday.
Honduran migrant Ana Zuniga, 23, said she saw migrants open a small hole in concertina wire at a gap on the Mexican side of the 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 three-year-old daughter Valery in her arms.
US military helicopters have been hovering over the Mexican side of the border, while US agents held vigil on foot beyond the wire fence in California.
The Border Patrol in San Diego said that pedestrian crossings have been suspended at the San Ysidro port of entry at both the east and west facilities.
The dramatic shutdown took place just days after US President Donald Trump threatened to close the "whole border" with Mexico if "it gets to a level where we're going to lose control or people are going to start getting hurt."
More than 5,000 migrants are camped in and around a sports complex in Tijuana after making their way through Mexico in recent weeks in a caravan.
Many are hoping 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 on Sunday called on each other to remain peaceful.
They appeared to pass through the Mexican police blockade with ease 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.
Irineo Mujica, who has accompanied the migrants as part of the aid group Pueblo Sin Fronteras, said the aim of Sunday's march was to make the migrants' plight more visible to the governments of Mexico and the US.
"We can't have all these people here," he said.
Mr Trump has repeatedly warned that the large group of migrants heading for the US included criminals and possibly terrorists - though there is no evidence to support that.
He has deployed around 9,000 US troops along the border in support of the CBP agents.
Mexico's incoming government has denied a report that it plans to allow asylum seekers to wait in the country while their claims move through US immigration courts.
"There is no agreement of any sort between the incoming Mexican government and the US government," Mexico's future interior minister Olga Sanchez said in a statement.
Social media users have reacted to the clashes between migrants and police at the US-Mexico border:
Several hundred Central American migrants stormed the international border crossing between Tijuana and California to pressure the US into hearing their asylum claims.
US agents began firing tear gas following clashes at the border after a small group of migrants kicked through part of a fence.
The migrants tried to protect themselves by covering their faces, with mothers holding their children close.
Most of the panicked migrants turned back and ran away amid the stinging gas as riot police began surrounding them.
Sky News' chief correspondent Stuart Ramsay said there were chaotic scenes as migrants got onto the train tracks which lead to the US, and there were "scuffles with the police" throughout.
"Some were able to get through a gap in the border fence - but on the other side the US border patrol were there and fired tear gas at them".
The migrants, mostly from Honduras, carried hand-painted American and Honduran flags, chanting: "We are not criminals! We are international workers!"
Some were seen scrambling up railway sidings in an attempt to reach the US border - with many young children at the centre of the mayhem.
Dozens of Mexican police with plastic riot shields stopped the migrants well ahead of the crossing on Sunday.
Honduran migrant Ana Zuniga, 23, said she saw migrants open a small hole in concertina wire at a gap on the Mexican side of the 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 three-year-old daughter Valery in her arms.
US military helicopters have been hovering over the Mexican side of the border, while US agents held vigil on foot beyond the wire fence in California.
The Border Patrol in San Diego said that pedestrian crossings have been suspended at the San Ysidro port of entry at both the east and west facilities.
The dramatic shutdown took place just days after US President Donald Trump threatened to close the "whole border" with Mexico if "it gets to a level where we're going to lose control or people are going to start getting hurt."
More than 5,000 migrants are camped in and around a sports complex in Tijuana after making their way through Mexico in recent weeks in a caravan.
Many are hoping 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 on Sunday called on each other to remain peaceful.
They appeared to pass through the Mexican police blockade with ease 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.
Irineo Mujica, who has accompanied the migrants as part of the aid group Pueblo Sin Fronteras, said the aim of Sunday's march was to make the migrants' plight more visible to the governments of Mexico and the US.
"We can't have all these people here," he said.
Mr Trump has repeatedly warned that the large group of migrants heading for the US included criminals and possibly terrorists - though there is no evidence to support that.
He has deployed around 9,000 US troops along the border in support of the CBP agents.
Mexico's incoming government has denied a report that it plans to allow asylum seekers to wait in the country while their claims move through US immigration courts.
"There is no agreement of any sort between the incoming Mexican government and the US government," Mexico's future interior minister Olga Sanchez said in a statement.
Social media users have reacted to the clashes between migrants and police at the US-Mexico border:
