What happens next?
The Department of Homeland Security says it is monitoring the caravan, promising to enforce U.S. immigration laws and accusing the migrants of having "the apparent intention of entering the United States illegally."
That is not what the caravan participants say they plan to do. They say that on Sunday they will seek asylum, following the legal process for asking for safe harbor from persecution in their home countries.
KPBS's Guerrero spoke to Jeimy Pastora Castro in Tijuana, outside the port of entry. Castro, who is from Honduras, was holding her 2-year-old daughter.
"I didn't come here to cross illegally," Castro said. "I came here for help, for the president to have a heart — since he's a human like us ... and was also created by God."
"Both U.S. law and a U.N. treaty on the rights of refugees require that asylum-seekers get a fair hearing," Guerrero notes. "Most of the people in this caravan come from Honduras, where a divisive presidential election has led to killings in the streets."
DHS says that in anticipation of asylum claims, extra immigration judges and attorneys are being sent to the southern U.S. border.
"For those seeking asylum, all individuals may be detained while their claims are adjudicated efficiently and expeditiously, and those found not to have a claim will be promptly removed from the United States," Secretary of Homeland Security Kirstjen Nielsen said.
The U.S. offers asylum to those with a "well-founded fear of persecution" in their home country, on the basis of their race, religion, nationality, political opinion or membership in a certain social group.
Central Americans fleeing violence have found it difficult to receive asylum based on gang violence waiting for them at home, as NPR's Eyder Peralta reported in 2016. The U.S. has tended to identify those people as fleeing crime, which is not grounds for asylum, rather than individual persecution.
And the Trump administration has been working to limit asylum further, as NPR's Joel Rose reported last month.
Attorney General Jeff Sessions, who has criticized "vague, insubstantial and subjective" asylum claims, is using his power to overturn judicial decisions in immigration court to reshape how those courts work.
But to qualify for asylum, or be rejected, a migrant must first be processed at the border.
As NPR's John Burnett reported last year, would-be asylum-seekers at the U.S.-Mexico border have described being turned away or threatened with deportation or told by officials that there is no such thing as asylum.
Turning away someone seeking asylum without considering their claim violates national and international law.
U.S. Customs and Border Protection denies that asylum-seekers are being turned away.
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