A federal judge on Saturday blocked the Trump administration from using an 18th century law known as the Alien Enemies Act to deport five Venezuelans, kicking off a blizzard of litigation over the controversial move even before the president has announced it.

President Donald Trump has widely signaled he would invoke the 1798 Act, last used to justify the internment of Japanese-American civilians during World War 2.

On Saturday, the American Civil Liberties Union and Democracy Forward filed an extraordinary lawsuit in federal court in Washington contending the order would identify a Venezuelan gang, Tren de Aragua, as a “predatory incursion” by a foreign government and seek to deport any Venezuelan in the country as a member of that gang, regardless of the facts.

James E. Boasberg, chief judge of the D.C. Circuit, agreed to implement a temporary restraining order preventing the deportation for 14 days under the act of the five Venezuelans who are already in immigration custody and believed they were being moved to be deported. Boasberg said his order was “to preserve the status quo.” Boasberg scheduled a hearing for later in the afternoon to see if his order should be expanded to protect all Venezuelans in the United States.

Hours later, the Trump administration appealed the initial restraining order, contending that halting a presidential act before it has been announced would cripple the executive branch.

If the order were allowed to stand, “district courts would have license to enjoin virtually any urgent national-security action just upon receipt of a complaint,” the Justice Department wrote in its appeal.

It said district courts might then issue temporary restraining orders on actions such as drone strikes, sensitive intelligence operations, or terrorist captures or extraditions. The court “should halt that path in its tracks,” the department argued.

The unusual flurry of litigation highlights the controversy around the Alien Enemies Act, which could give Trump vast power to deport people in the country illegally. It could let him bypass some protections of normal criminal and immigration law. But it would face immediate challenges along the lines of Saturday’s litigation because it has previously only been used during wartime.

The law requires a formal declaration of war before it can be used. But immigration lawyers were alarmed by a flurry of activity Friday night.

“Last night, it appears the government was preparing to deport a number of Venezuelans they had no legal authority to deport,” said Ahilan Arulanantham, an immigration lawyer in Los Angeles who filed two petitions to block deportations that night.