Thousands of asylum-seekers gathered at the U.S.-Mexico border as the Biden administration prepared Thursday to begin denying asylum to migrants who arrive at the border without first applying online or seeking protection in a country they passed through.

It marks a fundamental shift in immigration policy as the U.S. readies for the end of a key pandemic restriction.

Asylum-seekers have been showing up at the border in huge numbers in anticipation of this week’s end of the use of a restriction known as Title 42. That rule has allowed the government to quickly expel migrants to Mexico. U.S. officials warned of difficult days ahead as the program tied to the COVID-19 pandemic expires this week.

The rule announced Wednesday is part of new measures meant to crack down on illegal border crossings while creating new legal pathways. Families who cross the border will face curfews and monitoring; the head of household will wear an ankle bracelet as their cases are heard within 30 days.

But there’s also a plan to open 100 regional migration hubs across the Western Hemisphere and grant humanitarian parole to 30,000 people a month to enter the country from four countries.

U.S. officials have detailed steps they’ve taken, including increasing deportation flights, as they prepare for what many are expecting to be a substantial increase in migrants at the U.S.-Mexico border.

"Our plan will deliver results, but it will take time for those results to be fully realized," Homeland Security Secretary Alejandro Mayorkas warned.

Many migrants, spurred by concerns that it may soon become harder to stay in the U.S., were trying to cross before Title 42 expires and the new rule takes effect at the end of the day Thursday.

Under Title 42, border officials have quickly returned people – and they did so 2.8 million times since March 2020. But after the restrictions expire Thursday, migrants caught crossing illegally will not be allowed to return for five years. They can face criminal prosecution if they do.

At the Rio Grande in Matamoros on Wednesday, migrants arrived steadily. Many stripped down before descending the steep riverbank grasping plastic bags filled with clothes. They slowly waded into the river as more migrants arrived, some crossing themselves before following the line across the flowing border. One family swaddled a tiny baby inside an open suitcase. A man held it atop his head while another waded beside him as a precaution. Other children rode on shoulders. On the U.S. side, they scrambled up the bank, pausing to put on dry clothing, before carefully picking their way through the rows of concertina wire.

Fear and anguish

In Ciudad Juarez, migrants arrived this week in small groups by train or bus, leaving daily to surrender to the U.S. authorities.

Fran Tovar, a 30-year-old electrician from Venezuela who left two children behind to try to reach the U.S., was expelled from the U.S. on his first attempt. He was trying again 24 hours later, with the goal of crossing before Title 42’s use expires.

"There is fear and anguish," Tovar said Wednesday, adding that he has spent three months in Juarez trying to get an appointment through an app the U.S. has encouraged migrants to use to present themselves at a border entry point and seek admission.

Roughly 10,000 people were apprehended by Border Patrol on Tuesday, among the largest apprehensions in a single day, according to a U.S. official who was not authorized to speak publicly and spoke to The Associated Press on condition of anonymity. More than 27,000 people were in custody. Custody numbers vary as migrants are released or deported, but in March 8,600 people were in Border Patrol custody.

Miguel Meza, the head of migrant programs for Catholic Relief Services, which has 26 migrant shelters across Mexico, estimated that about 55,000 migrants were in the border cities across from the U.S. on Wednesday. The shelter space is "saturated," he said, and migrants were spilling into areas around them.

The measure announced Wednesday is a key part of the U.S. strategy to address border crossings that rose to all-time highs even with Title 42 in effect. While stopping short of a total ban, it imposes severe limitations on asylum for those crossing illegally who didn't first seek a legal pathway. It includes room for exceptions and does not apply to children traveling alone. It was first announced in February.

A federal appeals court prevented similar but stricter measures pursued by then-President Donald Trump in 2019 from taking effect.

Human rights groups said they plan to sue quickly.

"This rule will subject people to grave harm," said American Civil Liberties Union lawyer Katrina Eiland.

She said it would result in migrants stranded in northern Mexico. She said the rule was predicated on the idea migrants can get protection in another country or get an appointment online to seek asylum in the U.S. She said there are serious problems with both those options.

U.S. officials also said they planned to open regional hubs around the hemisphere, where migrants could apply to go to the U.S., Canada, or Spain. Two hubs were previously announced in Guatemala and Colombia. It's unclear where the other locations would be.

The administration officials spoke on the condition of anonymity to discuss ongoing border plans that were not yet public.

Most of the people going to the U.S.-Mexico border are fleeing persecution or poverty in their home countries. Migrants and groups who work with them noted the swirl of rumors and disinformation from smugglers that makes it hard for migrants to understand what to do.

In Matamoros, Carmen Josefina Characo Lopez said she arrived over a month ago and had been trying to use the U.S. government's app to schedule an appointment to seek asylum.

"People who just arrive start hearing the stories of others who have been here longer and they start getting alarmed. ‘Oh, you’ve been here for four months. Well, I just got here and I’m going to cross.’ And that’s where the dilemma is," she said.