WASHINGTON, Dec. 16 -- A U.S. federal judge ruled on Monday that the National Security Agency's bulk collection of domestic phone records is likely unconstitutional.

This is a first significant legal setback for the secret intelligence surveillance program since it was first revealed by defense contractor Edward Snowden in June.

In his 68-page opinion, Judge Richard Leon of the U.S. District Court in Washington D.C., found that the plaintiffs of the lawsuit against the Obama administration had demonstrated "a substantial likelihood" of the NSA's domestic phone scooping program violating the Fourth Amendment, which bans unreasonable searches and seizures.

"I cannot imagine a more indiscriminate and arbitrary invasion than this systematic and high-tech collection and retention of personal data on virtually every single citizen for purposes of querying it and analyzing it without judicial approval," wrote Leon.

Leon granted a request from activist Larry Klayman, one of the two plaintiffs, for a preliminary injunction to block the NSA's phone records collection. But the judge put off enforcing his order to allow for an appeal by the government.

This is the first time a court has ruled against the NSA's massive surveillance program on U.S. phones since June.

The American Civil Liberties Union and other groups have also brought separate lawsuits challenging the program, which the intelligence community and the Obama administration have defended as important to protect national security against terrorism.

"I have significant doubts about the efficacy of the metadata collection program as a means of conducting time-sensitive investigations in cases involving imminent threats of terrorism," he wrote. "The government does not cite a single instance in which analysis of the NSA's bulk metadata collection actually stopped an imminent attack, or otherwise aided the Government in achieving any objective that was time-sensitive in nature."

The judge even compared the NSA phone program to George Orwell' s 1984.

"The almost-Orwellian technology that enables the government to store and analyze the phone metadata of every telephone user is unlike anything that could be conceived in 1979," he wrote.

The government lawyers argued that a 1979 Supreme Court ruling in Smith v. Maryland case, found no search warrant was needed by police on phone surveillance. But Leon said such a ruling of over 30 years old may not be applicable to surveillance programs in recent years, as people in 2013 have "an entirely difference relationship with their phones" than 34 years ago.

"The ubiquity of phones has dramatically altered the quantity of information that is available and, more importantly, what that information can tell the government about people's lives," he wrote.

At a daily press briefing shortly after the ruling, White House spokesman Jay Carney referred the inquiries to the Justice Department. The Justice Department spokesman Andrew Ames said the department is still reviewing the federal court's decision.