The migrants' arrival has been met by some opposition from people in Tijuana, where many shelters were already at capacity, and on the USA side of the border by increased security measures that include members of the military deployed there by President Donald Trump. Close to 3,000 migrants took Mexico up on the offer. Mexican police officers in riot gear stopped many, but those who continued toward the fence met USA border agents, who sprayed some with tear gas.
Later, immigrants again approached the border in groups and were met by a further volley of canisters emitting large clouds of gas. They appeared to easily pass through without using violence, and some of the migrants called on each other to remain peaceful.
Fani Caballero, 32, a migrant from Honduras, sat by the train tracks, within sight of US agents on the other side of the border fence.
On Twitter, Sen. Brian Schatz, D-Hawaii, wrote that the "tear gas across the border against unarmed families is a new low".
Numerous migrants had gathered earlier in the day to protest against being forced to stay in Tijuana instead of being allowed to cross and seek asylum in the US.
As they reached the border area, some protesters split off toward multiple locations, CBP said. Many hope to apply for asylum in the USA, but agents at the San Ysidro entry point are processing fewer than 100 asylum petitions a day.
To enter the USA legally, migrants must formally apply for asylum, but the wait for those requests to be processed can last up to a year.
Image Mexico said it would not send military forces to control migrants
Stephanie Leutert, director of the Mexico Security Initiative at the University of Texas at Austin, described the Remain in Mexico plan as a strategy to take away the ability of migrants to live and work in the USA while cases are processed.
Demonstrators' show of solidarity came as U.S. Border Patrol agents fired tear gas at Central American migrants after as many as 500 people rushed police blockades on the Mexican side of the southern border in an apparent bid to cross into the U.S.
But Mexico's incoming government, which assumes power December 1, denied that it is willing to let USA asylum-seekers stay there pending the outcome of their cases in US immigration courts, which could take years.
Amnesty International said what happened Sunday was predictable after Trump ordered the troops to the border and said he told them that if a migrant throws a rock, "consider it a rifle".
"People had thought that they were going to open the gates, but that was a lie", Caballero said.
On Sunday, Trump took to Twitter Sunday to express his displeasure with the caravans in Mexico.
Late Sunday evening, hundreds of illegal aliens attempted to rush the United States-Mexico border, something they talked about a few days ago. During his presidential campaign, he promised he'd have Mexico pay for the wall.More news: Chinese e-commerce sites ditch Dolce & Gabbana in ad backlash