The Trump administration faced the first direct legal challenge to its policy of sending migrants to the U.S. military base at Guantánamo Bay, Cuba, for continued immigration detention with a lawsuit filed on Saturday by a coalition of human rights and immigrant advocacy organizations.

“Plaintiffs seek this court’s intervention to put a stop to these cruel, unnecessary and illegal transfers to and detention at Guantánamo,” the newly filed complaint said.

The plaintiffs, led by the American Civil Liberties Union, are for now seeking a judicial stay to block the transfer of 10 migrants whom the coalition signed up to represent. But it appears to lay the groundwork to seek a potential broader order against the transfer policy, which has raised many novel legal issues.

The 10 migrants named in the lawsuit each has final removal orders, it said, and comes from countries including Afghanistan, Bangladesh, Pakistan and Venezuela. The lawsuit asserts that none are gang members, and some have been specifically threatened with transfer to Guantánamo.

“In attempting to justify the transfers, the government has claimed that the individuals it sent to Guantánamo are members of gangs and dangerous criminals — the ‘worst of the worst,’” the complaint said, citing a remark in January by Defense Secretary Pete Hegseth.

It continued: “That characterization is patently false. It is also legally irrelevant because the government lacks statutory authority to send any immigration detainees from the United States to Guantánamo.”

The Justice Department press office did not immediately respond to a request for comment.

The lawsuit is not the first to challenge aspects of President Trump’s policy. Last month, a judge prevented the government from moving three Venezuelan men who were being held in immigration detention in New Mexico to the base, and a group of legal aid organizations sued the administration asking that migrants taken there have access to lawyers.

Neither of those cases, however, directly addressed the legality of the overall policy. The new lawsuit claims that it exceeds the government’s authority under the Immigration and Nationality Act to take the migrants to Cuban soil, and that the government has no statutory authority to detain people outside the United States for immigration purposes.

Calling such transfers “arbitrary and capricious,” the lawsuit also claims that the policy violates the Administrative Procedure Act and the migrants’ due process rights.

“It’s not just that it’s illegal, but wholly illogical from a cost standpoint, something this administration supposedly cares about,” said Lee Gelernt of the American Civil Liberties Union, who is the lead lawyer in the lawsuit. “The administration has had its Guantánamo photo op moments, and now it’s time to move on.”

It has not been clear whether there is any concrete policy advantage to the cost that taxpayers are incurring for flying migrants to the remote island base rather than housing them more cheaply on U.S. soil until directly deporting them to their home countries.

But the operation has generated stories that could send a deterrent message — a purpose Mr. Hegseth appeared to allude to last week when he visited the base with a former colleague from Fox News.

“The message is clear: If you break the law, if you are a criminal, you can find your way at Guantánamo Bay,” Mr. Hegseth told Fox. “You don’t want to be at Guantánamo Bay, which is where we housed Al Qaeda after 9/11.”

Mr. Trump directed the U.S. military and the Homeland Security Department on Jan. 29 to prepare to expand a migrant operations center at Guantánamo Bay, saying it would “provide additional detention space for high-priority criminal aliens unlawfully present in the United States.”

Soon after, the military began transporting migrants to the base on what became near daily flights from an immigration site in El Paso. Despite the Trump administration’s portrayal of them as criminals, only some of the migrants who have been identified as being transferred to the base have had criminal records.

The first 178 migrants taken there were all citizens of Venezuela, a country to where deporting people had been difficult because of a breakdown in relations between its authoritarian government and the United States.

However, the Trump administration has persuaded Venezuela to begin taking its people back. On Feb. 20, it abruptly cleared out the detention operation, sending 177 migrants to Honduras where they were picked up by a Venezuelan plane and taken home. (One man had earlier been transferred back to the United States.)

Then, in a series of flights starting on Feb. 23, the administration began sending more migrants there, this time from a spectrum of other countries including Honduras, Colombia, El Salvador, Guatemala and Ecuador, according to a document seen by The New York Times. They ranged in age from 23 to 62.

As of Friday morning, the military was holding 26 migrants in a dormitory-style building handled by the Coast Guard, where it has been housing those deemed to be “lower risk,” and 17 men in a war-on-terror prison called Camp 6, where it has sent those deemed “high risk,” according to a defense official who was not authorized to speak about the matter by name.

Nine migrants were sent back to the United States this week. Another flight arrived Friday afternoon, but the number migrants who were on it and which of the two holding facilities they were sent to is unclear.

The new lawsuit is likely to be handled by Judge Carl Nichols of the Federal District Court in Washington. Judge Nichols, a Trump appointee, was earlier assigned the legal access suit, and the coalition filed the new lawsuit as a related matter. Mr. Gelernt is also the lead lawyer in the earlier case.