The judge in Washington issued a preliminary injunction blocking the U.S. government's massive collection of phone records, but stayed it pending appeal.
"We've seen the opinion and are studying it," U.S. Justice Department spokesman Andrew Ames said. "We believe the program is constitutional as previous judges have found. We have no further comment at this time."
The program's actions are approved by a special panel, the Foreign Intelligence Surveillance Court, made up of federal judges.
U.S. District Judge Richard J. Leon, a George W. Bush appointee, said a suit by Larry Klayman, the founder of Freedom Watch, has "demonstrated a substantial likelihood of success" on the basis of Fourth Amendment, the Washington Post reported.
The amendment bans unreasonable searches.
Though Leon granted Klayman's request for a preliminary injunction, he stayed his order pending an appeal by the Obama administration because of the "significant national security interests at stake in this case and the novelty of the constitutional issues," the Post reported
Klayman's challenge to the National Security Agency's surveillance program is one of a series of cases filed in federal court since the leaks this summer by former NSA contractor Edward Snowden, who has temporary political asylum in Russia.
The suit by Klayman and several other plaintiffs targets PRISM, a secret electronic surveillance data mining program operated by the National Security Agency since 2007.