Packed into five old school buses, hundreds of Central American migrants have arrived at the US border on Sunday for a rally, to be followed by a planned mass attempt to apply for asylum in a direct challenge to the Trump administration.
The migrants, many travelling with children, left a downtown Tijuana shelter where they had been staying. Police with flashing lights on Sunday escorted the buses to a cross-border rally at a Pacific Ocean beach, with supporters gathering on both sides of security fencing.
US President Donald Trump and members of his cabinet have been tracking the caravan of migrants, calling it a threat to the US since it started on March 25 in the Mexican city of Tapachula, near the Guatemala border.
Attorney-General Jeff Sessions has called the caravan "a deliberate attempt to undermine our laws and overwhelm our system", pledging to send more immigration judges to the border to resolve cases if necessary.
Trump administration officials have railed against what they call America's "catch-and-release" policies that allow people requesting asylum to be released from custody into the US while their claims make their way through the courts, a process that can last a year.
The arrival at San Diego's San Ysidro border crossing, the nation's busiest, marked the end of a month-long journey by foot, freight train and bus for the migrants, many of whom said they feared for their lives in their violence-racked home countries.
But the travellers faced an uncertain future as they prepared to turn themselves in and ask for asylum. US immigration lawyers warned them they face possible separation from their children and detention for many months.
Administration officials and their allies claim that asylum fraud is growing and that many who seek it are coached on how to do so.
Asylum seekers are typically held for up to three days at the border and then turned over to US Immigration and Customs Enforcement. If they pass an asylum officer's initial screening, they may be detained or released into the US with ankle monitors.