DETROIT — A federal judge ruled yesterday that the government’s warrantless wiretapping program is unconstitutional and ordered an immediate halt to it.

U.S. District Judge Anna Diggs Taylor in Detroit became the first judge to strike down the National Security Agency’s program, which she says violates the rights to free speech and privacy as well as the separation of powers enshrined in the Constitution.

“Plaintiffs have prevailed, and the public interest is clear in this matter. It is the upholding of our Constitution,” Judge Taylor wrote in her 43-page opinion.

The American Civil Liberties Union filed the lawsuit on behalf of journalists, scholars, and lawyers who say the program has made it difficult for them to do their jobs. They believe many of their overseas contacts are likely targets of the program, which involves secretly listening to conversations between people in this country and people in other countries.

The government argued that the program is well within the president’s authority but said proving that would require exposing state secrets.

The ACLU said the state-secrets argument was irrelevant because the Bush administration had already publicly disclosed enough information about the program for Judge Taylor to rule on the case.

“By holding that even the president is not above the law, the court has done its duty,” the ACLU’s associate legal director and the lead attorney for the plaintiffs, Ann Beeson, said.

The NSA had no immediate comment on the ruling.

Judge Taylor dismissed a separate claim by the ACLU over data-mining of phone records by the NSA. She said not enough had been publicly disclosed about that program to support the claim, and further litigation could jeopardize state secrets.

Ms. Beeson predicted the government would appeal the ruling and request that the order to halt the program be postponed while the case makes its way through the system. She said the ACLU had not yet decided whether it would oppose such a postponement.