EL PASO - The Biden administration Friday began a transition from the pandemic-era border policy that swiftly expelled migrants who entered the United States illegally to a more complex system that encourages applications for legal entrance.

The Title 42 policy, which launched during the Trump administration and resulted in the expulsions of more than 2.6 million migrants from the United States during the past three years, expired at the end of the day Thursday.

The Biden administration says its new system will make it easier for authorities to deport asylum seekers who cross illegally, while expanding opportunities for migrants to reach the United States legally if they apply via an app.

Migrants who ignore the rules could be quickly deported to Mexico or their home country, with consequences that did not apply during the Title 42 expulsions. Federal officials say people who cross illegally could be deported, barred for five years and prosecuted for the crime of illegal entry.

At midnight, Homeland Security Secretary Alejandro Mayorkas tweeted a warning that 24,000 Border Patrol agents and officers and thousands of troops and contractors are on hand to enforce the policy. "Do not believe the lies of smugglers," he said. "The border is not open."

Late Thursday, a federal judge in Florida temporarily blocked the Biden administration from quickly releasing migrants from crowded Border Patrol holding facilities.

U.S. District Judge T. Kent Wetherell II wrote in his ruling that the border had been "out of control" for two years and said that the president and Congress had failed to fix it. He said he would not condone a new emergency policy Border Patrol Chief Raul Ortiz issued this week allowing for the release of some migrants into the United States, since it was similar to a policy he had rejected as unlawful in March.

The Biden administration had warned the judge, a Trump appointee, that border facilities could become dangerously overcrowded if he blocked the emergency releases, but Wetherell wrote that the administration's "doomsday rhetoric rings hollow."

Department of Homeland Security spokeswoman Marsha Espinosa emailed a statement on behalf of Customs and Border Protection saying the agency would comply with the court order. The statement called it "a harmful ruling that will result in unsafe overcrowding at CBP facilities."

The mere anticipation of the policy's end triggered a race to the border this week: Tens of thousands of migrants have been crossing, with some saying they fear they're more likely to be deported after the measure lifts.

Biden administration officials said in recent days that they were bracing for a historic surge of migrants along the U.S.-Mexico boundary, from Southeast Texas to Southern California, and the president himself warned that the coming weeks could be chaotic.

Migrant crossings are already at a record high in anticipation of the changing border policies.

Officials also said they were confident that new policies would work over time.

"We are clear-eyed about the challenges we are likely to face in the days and weeks ahead, and we are ready to meet them," Mayorkas said at the White House.

Unlawful crossings have topped 10,000 per day this week, the highest levels ever. The Border Patrol is already on track to exceed the 2.1 million apprehensions last year, federal data show.

The rush prompted Ortiz on Wednesday to issue the emergency policy, which Wetherell temporarily blocked, allowing supervisors in overcrowded areas to quickly release migrants using an authority he called "Parole with Conditions." Migrants would be vetted and directed to report to U.S. immigration authorities in their destination cities within 60 days.

More than 27,000 migrants were in Customs and Border Protection custody at one point this week, triple the official capacity. Eight of nine border patrol sectors said their holding cells were stretched beyond their limits.

The Ortiz memo prompted Florida's Republican Attorney General Ashley Moody to file a federal lawsuit urging a judge in that state to halt the releases, saying the new policy was similar to another a judge had struck down in March.

Moody accused the Biden administration in a statement of attempting to "illegally dismantle our nation's border security system."

The Biden administration said the releases respond to an "imminent crisis" and that halting the releases could "have extremely dire and catastrophic consequences." Crowding could endanger migrants and staff and lead to the spread of contagious diseases such as measles, officials said in court filings.

And the number of detainees could reach 45,000 migrants by month's end, officials said.

In his ruling Thursday, Wetherell said the new Ortiz policy was nearly indistinguishable from the one he had rejected earlier, and scheduled a hearing for May 19. The Biden administration could seek an emergency stay of his decision, the judge wrote.

This week, thousands of migrants forded the Rio Grande into the Brownsville, Tex., area, or arrived elsewhere. The locations included a dusty strip of U.S. land between the riverbanks and the border wall east of downtown El Paso, more than 650 miles away.

In Brownsville on Thursday, migrants streamed across the border to enter the U.S. before new immigration rules took effect. A half-dozen Mexican migration officials on the riverbank near the international bridge could not hold back the flow.

Carolina, a 36-year-old Venezuelan mother who declined to give her full name, stood with her two young sons on the bank of the river in the Mexican border city of Matamoros, scouting the scene. She knew the river crossing could be dangerous; some people had drowned.

"You have to trust in God," she said. She believed the end of Title 42 would make it harder to get into the United States.

A Mexican immigration agent threatened to report her to child-protection services for putting her children in danger.

She ignored him.

"My daughter already crossed!" she said. She turned to her 11- and 13-year-old sons and then gazed back at the river. "There's your father!" she cried, spotting a man in a black sleeveless T-shirt wading through the river toward them.

She and the boys scrambled down the embankment and the father scooped up his young son. They soon joined dozens of people turning themselves into U.S. officials on the Brownsville side.

Every few minutes the Border Patrol broadcast a message in Spanish, warning migrants not to cross and saying they would be deported if they did.

"Attention, attention, stay in Mexico," blared the announcement. Few paid heed.

Mata reported from El Paso, Sheridan from Matamoros, Mexico, and Sacchetti and Miroff from Washington.