On Saturday, the administration published an executive order invoking the law, the Alien Enemies Act of 1798, to target Venezuelan gang members in the United States.

But shortly after the announcement, James E. Boasberg, a federal judge in Washington, D.C., said he would issue a temporary order blocking the government from deporting any immigrants under the law.

In a hastily scheduled hearing, he said he did not believe the law offered grounds for the president’s action, and he ordered any flights that had departed with Venezuelan immigrants under the executive order to return to the United States “however that’s accomplished — whether turning around the plane or not.”

“This is something you need to make sure is complied with immediately,” he directed the government.

Lee Gelernt, a lawyer for the American Civil Liberties Union, which sued over the executive order, said in an interview after the hearing that he believed two flights were “in the air” on Saturday evening.

During the hearing, Judge Boasberg said he was ordering the government to turn flights around given “information, unrebutted by the government, that flights are actively departing.”

A lawyer representing the government, Drew Ensign, told the judge that he did not have many details to share and that describing operational details would raise “national security issues.”

After the hearing, the government filed an appeal. In a statement late Saturday, Attorney General Pam Bondi said the judge had put “terrorists over the safety of Americans” and placed “the public and law enforcement at risk.”

“The Department of Justice is undeterred in its efforts to work with the White House, the Department of Homeland Security, and all of our partners to stop this invasion and Make America Safe Again,” she added.

The president’s order, dated Friday, declares that Venezuelans who are at least 14 years old, in the United States without authorization and part of the Tren de Aragua gang are “liable to be apprehended, restrained, secured and removed.”

The Alien Enemies Act allows for summary deportations of people from countries at war with the United States. The law, best known for its role in the internment of Japanese Americans during World War II, has been invoked three times in U.S. history — during the War of 1812, World War I and World War II — according to the Brennan Center for Justice, a law and policy organization.

Hours before the White House published its proclamation, the American Civil Liberties Union filed a lawsuit on behalf of five Venezuelan men seeking to block the president from invoking the law. All five men were accused of having links to Tren de Aragua but deny that they are in the gang, Mr. Gelernt said. One of the men was arrested, the lawsuit said, because an immigration officer “erroneously” believed he was a member of Tren de Aragua because of his tattoos.

Judge Boasberg initially issued a limited order on Saturday blocking the government from deporting the five men.

The Trump administration promptly filed an appeal of the order, and the A.C.L.U. asked the judge to broaden his order to apply to all immigrants at risk of deportation under the Alien Enemies Act. At the hearing Saturday evening, Judge Boasberg said he would issue a broader order applying to all “noncitizens in U.S. custody.”

In the lawsuit, lawyers for the American Civil Liberties Union wrote that the Venezuelans believed that they faced an immediate risk of deportation. “The government’s proclamation would allow agents to immediately put noncitizens on planes,” the lawsuit said, adding that the law “plainly only applies to warlike actions” and “cannot be used here against nationals of a country — Venezuela — with whom the United States is not at war.”

The judge agreed, saying that he believed the terms “invasion” and “predatory incursion” in the law “really relate to hostile acts perpetrated by enemy nations.”

The White House and the Homeland Security Department, which runs the nation’s immigration system and was named in the lawsuit, did not immediately respond to requests for comment.

Noah Feldman, a constitutional law professor at Harvard, said the fate of the case, which could ultimately wind up at the Supreme Court, would hinge on “how much deference the courts pay to the president’s determination that there’s a threatened incursion.” Judges would have to make that determination “without a lot of precedent,” Professor Feldman added.

President Trump, who campaigned last year on a promise to initiate the largest deportation operation in U.S. history, has often referred to the arrivals of unauthorized immigrants as an “invasion.” One of the first executive orders he issued after returning to the White House was titled, “Protecting the American People Against Invasion.”

His proclamation invoking the Alien Enemies Act appeared to be narrowly focused on Tren de Aragua, a gang that emerged from a Venezuelan prison and grew into a criminal organization focused on sex trafficking, drug dealing and human smuggling.

But if the Trump administration’s interpretation of the law is ultimately upheld, it could empower the administration to remove other immigrants age 14 or older without a court hearing. That would enable the extraordinary move of arresting, detaining and deporting immigrant minors without the due process afforded to immigrants for decades.

Skye Perryman, president of Democracy Forward, a legal group that joined the A.C.L.U. in submitting the challenge to the executive order, said in a statement that Saturday was a “horrific day in the history of the nation, when the president publicized that he was seeking to invoke extraordinary wartime powers in the absence of a war or invasion.”

Zolan Kanno-Youngs and Glenn Thrush contributed reporting.