Dozens of immigrants lined up Tuesday at a major Mexico border crossing, waiting to learn how the Trump administration's plans to end most asylum protections would affect their hopes of taking refuge in the United States.

The administration announced the plans a day earlier, reversing decades of U.S. policy in its most forceful attempt yet to slash the number of people seeking asylum in America. The new rule would cover countless would-be refugees, many of them fleeing violence and poverty in Central America. It is certain to face legal challenges.

Derek Mbi of Cameroon was among several dozen migrants who gathered in Tijuana, where he waited to hear how many people, if any, the U.S. would allow in for processing. He arrived in Tijuana about a month ago, and there are more than 8,100 people ahead of him on the waiting list.

FILE - In this April 1, 2019, file photo, Attorney General William Barr attends the 2019 Prison Reform Summit and First Step Act Celebration in the East Room of the White House in Washington. About 2,000 inmates are expected to be released from federal prisons across the U.S. this coming week as part of a sweeping criminal justice overhaul that was signed into law last year. The Justice Department and the federal Bureau of Prisons are working to create risk assessment tools and other measures that would help reduce the federal prison population while easing the transition for inmates back into the community. (AP PhotoSusan Walsh, File)

FILE - In this April 1, 2019, file photo, Attorney General William Barr attends the 2019 Prison Reform Summit and First Step Act Celebration in the East Room of the White House in Washington. About 2,000 inmates are expected to be released from federal prisons across the U.S. this coming week as part of a sweeping criminal justice overhaul that was signed into law last year. The Justice Department and the federal Bureau of Prisons are working to create risk assessment tools and other measures that would help reduce the federal prison population while easing the transition for inmates back into the community. (AP PhotoSusan Walsh, File)

Processing new arrivals has ground to a virtual halt to in recent days, down from an average of about 40 names a day.

Mbi, 29, joined a wave of Cameroonians who fled fierce government oppression against their country's English-speaking minority by flying to Ecuador, which does not require a visa. From there, he traveled for months by bus and on foot through seven other countries to reach Tijuana.

Mbi learned about the new policy but mistakenly believed that it applied only to Central and South Americans.

Attorney General William Barr, left, and President Donald Trump turn to leave after speaking about the 2020 census in the Rose Garden of the White House, Thursday, July 11, 2019, in Washington. (AP PhotoAlex Brandon)

Attorney General William Barr, left, and President Donald Trump turn to leave after speaking about the 2020 census in the Rose Garden of the White House, Thursday, July 11, 2019, in Washington. (AP PhotoAlex Brandon)

"When I got to Ecuador, I had a language barrier, said Mbi, who hopes to settle with a friend in Texas. "If I can go to the United States or Canada, I will go."

For now he is sharing a one-bedroom apartment with 13 Cameroonians in Tijuana and scraping by with odd jobs, like peeling tomatoes at open-air markets. He said many companies refused to hire him because his short-term transit permit in Mexico does not allow him to work.

Mbi declined to discuss why he fled Cameroon.

According to the plan published in the Federal Register , migrants who pass through another country — in this case, Mexico — on their way to the U.S. will be ineligible for asylum. The rule also applies to children who have crossed the border alone.

The vast majority of people affected by the rule are from Central America. But sometimes migrants from Africa , Cuba or Haiti and other countries try to come through the U.S.-Mexico border, as well.

There are some exceptions, including for victims of human trafficking and asylum-seekers who were denied protection in another country. If the country the migrant passed through did not sign one of the major international treaties governing how refugees are managed (though most Western countries signed them) a migrant could still apply for U.S. asylum.

Mexican Foreign Relations Secretary Marcelo Ebrard said Monday that his country "does not agree with any measure that limits access to asylum." Mexico's asylum system is also currently overwhelmed.

Trump administration officials say the changes are meant to close the gap between the initial asylum screening that most people pass and the final decision on asylum, which most people are denied.

Attorney General William Barr said that the United States is "a generous country but is being completely overwhelmed" by the burdens associated with apprehending and processing hundreds of thousands of migrants at the southern border.

He also said the rule is aimed at "economic migrants" and "those who seek to exploit our asylum system to obtain entry to the United States."

But immigrant rights groups, religious leaders and humanitarian groups have said the administration's policies amount to a cruel effort to keep immigrants out of the country. Guatemala, Honduras and El Salvador are poor countries, often wracked by violence.

"This is yet another move to turn refugees with well-founded fears of persecution back to places where their lives are in danger. In fact, the rule would deny asylum to refugees who do not apply for asylum in countries where they are in peril," said Eleanor Acer of Human Rights First.

ACLU attorney Lee Gelernt, who has litigated some of the major challenges to the Trump administration's immigration policies, said the rule was unlawful and the group planned to sue.

"The rule, if upheld, would effectively eliminate asylum for those at the southern border," he said. "But it is patently unlawful."

The new rule also will apply to the initial asylum screening, known as a "credible fear" interview, at which migrants must prove they have credible fears of returning to their home country. It applies to migrants who are arriving to the U.S., not those who are already in the country.

Along with the administration's recent effort to send asylum seekers back over the border , Trump has tried to deny asylum to anyone crossing the border illegally and restrict who can claim asylum. The attorney general recently tried to keep thousands of asylum seekers detained while their cases play out.

Nearly all of those efforts have been blocked by courts.

Tens of thousands of Central American migrant families cross the border each month, many claiming asylum. Border facilities have been dangerously cramped and crowded well beyond capacity. The Department of Homeland Security's watchdog found fetid, filthy conditions for many children. And lawmakers who traveled there recently decried conditions .