TIJUANA, Mexico — Hundreds of migrants who have traveled en masse through Mexico in the hope of getting safe passage into the United States, drawing a furious response from President Trump, began the final leg of their journey early on Sunday.
Joined by supporters and dozens of members of the news media, the migrants gathered in a park on the Pacific Ocean at about 10 a.m. local time for an event that was partly a celebration of the end of arduous weeks of travel and partly a protest to demand better treatment of refugees across the hemisphere.
At roughly the same time, scores of supporters, some of whom had walked from as far as Los Angeles, gathered about 30 yards away, just north of the fence separating the United States from Mexico.
The gathering on the Mexican side of the border was a prelude to the group’s final, decisive act. About 1 p.m. local time, accompanied by volunteer lawyers, paralegals and other advocates, the migrants were to begin marching to a border crossing between downtown Tijuana and southern San Diego.
There, about 180 people — mainly women and children fleeing poverty and violence in Central America — planned to present themselves to United States border officials and request asylum.
It was a day of emotional reckoning as the migrants prepared to part ways and to take their chances with the immigration authorities.
During the trip from the southern border of Mexico, friendships had been forged and romances kindled. In the oceanfront park on Sunday morning, before the larger group arrived, four migrant couples were married in a small ceremony attended by several of their friends. The weddings were conducted by a pastor from Chicago who wore monarch butterfly wings strapped to her back.
The monthlong odyssey began on March 25 in Tapachula, on Mexico’s border with Guatemala, following a familiar pattern. Groups of a few hundred migrants have traveled north together through Mexico as a “caravan” several times in recent years, their numbers providing protection from thieves, extortionists and rapists.
This time, however, the caravan quickly grew to upward of 1,200 people — far larger than its organizers, the transnational advocacy group Pueblo Sin Fronteras, had anticipated.
And there was another crucial difference: While past migrations had barely registered outside Mexico, this one caught the eye of Mr. Trump. In early April, Mr. Trump unleashed a barrage of tweets warning of the supposed threats the caravan posed to the security and sovereignty of the United States and accusing Mexico of doing little to stop illegal migration.
Undeterred, the caravan continued north in a herky-jerky manner, its participants traveling by any means possible, including on foot and by hitchhiking, but also atop freight trains and buses. Hundreds dropped out along the way, choosing to travel on their own or to stay in Mexico.
Remnants of the original group, now numbering about 300, began to arrive at the northern border early this past week and gradually converged on Tijuana. They crammed into overtaxed migrant shelters and gathered themselves for the caravan’s last act.
Many of those who will not seek asylum in the United States Sunday, hope to apply for protection in Mexico or elsewhere, or apply for asylum in the United States at a later date.
With the migrants on the doorstep of the United States, Mr. Trump, in a tweet, ratcheted up his rhetoric, vowing “not to let these large Caravans of people into our Country.” Attorney General Jeff Sessions called the caravan “a deliberate attempt to undermine our laws and overwhelm our system.”
In a statement Saturday, Rodney S. Scott, San Diego Chief Patrol Agent of Customs and Border Protection, said that groups of people associated with the caravan have illegally entered the United States by climbing over a metal fence. He said people who enter the country illegally will be referred for prosecution.
“To anyone that is associated with this caravan, think before you act,’’ the statement read. “If anyone has encouraged you to illegally enter the United States, or make any false statements to U.S. government officials, they are giving you bad advice and they are placing you and your family at risk.”
But the Trump administration’s aggressive response to the caravan has only seemed to embolden its organizers and to mobilize supporters. In recent days, scores of volunteer immigration lawyers and paralegals — from San Diego, Los Angeles, San Francisco, Las Vegas and elsewhere — have converged on Tijuana to help.
On Friday and Saturday, the lawyers met migrants individually to review their asylum requests to enter the United States.
To qualify, applicants must prove they have been persecuted or fear persecution based on their race, religion, nationality, political belief or membership in a particular group.
Those migrants deemed to have solid claims of persecution were encouraged to apply for asylum on Sunday. The lawyers advised others to seek protection in Mexico or elsewhere in Latin America.
People who request protection at a United States entry point must first be referred to an asylum officer for a screening, known as a credible-fear interview. If the officer finds that an applicant has a chance of proving fear of persecution, the person must then present his or her case before a judge.
While awaiting their court date, applicants could be released, albeit with an ankle-bracelet monitor. Those who do not pass their interview, or who do not qualify for one, could face months in detention and eventual deportation.
“We’re only sending people who we think will pass the credible-fear interview,” said Nicole Ramos, a volunteer immigration lawyer helping the caravan.
The Trump administration has sought to make it more difficult to apply for and to receive asylum, and immigrants’ advocates say that the number of asylum seekers released while their cases are pending has plunged since Mr. Trump took office.
A lawsuit filed in March by the American Civil Liberties Union and other groups accused the Trump administration of detaining asylum seekers to deter them and others from seeking sanctuary.
Ms. Ramos said she feared that with all the international attention on the caravan, the White House might be even less inclined to show mercy, even to those who pass their credible-fear interviews.
“They could use it as an opportunity to make an example out of people,” she said. “They’re assuming that people are breaking the law.”