Pandemic-era rules that have allowed US border guards to summarily expel hundreds of thousands of would-be asylum-seekers expired Friday, setting up an uncertain future for migrants and inflaming America's always-churning immigration debate.

Tens of thousands of people were expected to try to cross into the United States over the coming days, hoping to escape the poverty and criminal gangs that wrack their own countries.

But in an effort to avoid a surge, President Joe Biden's administration has put in place rules that raise the bar for anyone claiming refuge.

"Starting tonight, people who arrive at the border without using a lawful pathway will be presumed ineligible for asylum," Homeland Security Secretary Alejandro Mayorkas said.

"We have 24,000 Border Patrol Agents and officers at the Southwest Border and have surged thousands of troops and contractors, and over a thousand asylum officers to help enforce our laws."

- 'Political football' -

For more than three years the 2,000-mile (3,200-kilometer) frontier with Mexico has been regulated by Title 42, a health provision put in place to keep Covid infections at bay by refusing people entry.

But with the formal ending of the health emergency, that rule expired at midnight East Coast time (0400 GMT).

Asylum claims are now permitted again, but must in most cases be lodged before arriving at the border -- on pain of rapid expulsion.

Asylum-seekers are required to book interviews via a smartphone app -- though users report it is glitchy and presents a hurdle for those without working phones or wifi.

The Biden administration is trying to walk a tightrope between the humanitarian principles of his own Democratic Party, and avoiding the looped footage of hundreds of people pouring over the border.

Biden's Republican Party opponents have seized on what they say is an "invasion."

Senator Ted Cruz of Texas told reporters in Brownsville there were 22,000 people camping just on the other side of the frontier from this southernmost Texas city alone.

"And I have to say I am angry, because this is deliberate, this was a decision that was made by President Joe Biden and (Vice President) Kamala Harris and congressional Democrats to open up the border to what is nothing less than an invasion," he said.

Senator Roger Marshall of Kansas told Fox News the border situation was a "disaster."

"The number one threat to our national security is right here in southern Texas, all the way to Arizona," he said. "This is not the America that I grew up in."

Democratic congressman Mike Levin of California accused Republicans of playing "political football" with the country's broken immigration system.

They are more interested in "scor(ing) points than actually doing the work to fix it," he said.

- 'Only God knows' -

AFP reporters in Brownsville said there were dozens of police cars deployed on the US side of the bridge that connects the city to its Mexican neighbor Matamoros.

Heavy earth-moving equipment could be seen a little further on, with personnel readying the ground to install barbed wire.

On the streets of the city itself, Gabriel Landaeta, 22, was among those sleeping rough.

"If someday someone makes a documentary, let them put that the Venezuelan with a good heart came here looking for happiness," he told AFP.

In broken English, he and others try to find out what is happening.

"Title 42 is ending," a police officer tells them.

In El Paso, hundreds of people who passed into the country through a legitimate border gate on Thursday had been processed and allowed to lodge their initial asylum claim.

Many others were being held back by Texas National Guardsmen who stopped them from coming through the border.

And there was apparent confusion among rank-and-file border patrol officers about exactly what will happen in the coming hours and days.

"We don't know," said one when asked how they would handle migrants who made it through.

In Ciudad Juarez, Agustin Sortomi said he, his wife and two children had tried several times to surrender to US authorities but had been turned away.

"A lot of people are already coming from there saying that they closed the doors and they won't let anyone through. I don't know what to do," he told AFP.

"We haven't realized our dream. Only God knows when we will."

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