WASHINGTON - The US government's gathering of Americans' phone records is likely unlawful, a judge ruled on Monday, raising "serious doubts" about the value of the National Security Agency's so-called metadata counter terrorism program.

"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," US District Judge Richard Leon, appointed by Republican President George W. Bush in 2002, wrote in a 68-page ruling.

The US Department of Justice said it was reviewing the ruling in a case brought by Larry Klayman, a conservative lawyer, and Charles Strange, described in court documents as the father of a cryptologist technician for the NSA who was killed in Afghanistan in 2011. The judge ordered the government to stop collecting data about the two plaintiffs, who were Verizon Communications Inc customers. Verizon declined comment.

"We believe the program is constitutional as previous judges have found," Department of Justice spokesman Andrew Ames said in a statement.

Leon suspended enforcement of his injunction against the program "in light of the significant national security interests at stake in this case and the novelty of the constitutional issues" pending an expected appeal by the government. A US official said an appeal was likely.

Leon expressed skepticism of the program's value, writing that the government could not cite a single instance in which the bulk data actually stopped an imminent attack.

"I have serious 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.

That is important, he added, because for the program to be constitutional, the government must show its effectiveness outweighs privacy interests.

Leaks

Former NSA contractor Edward Snowden revealed the massive phone record collection to US and British media in June. Documents provided by Snowden showed that a US surveillance court had secretly approved the collection of millions of raw daily phone records in America, such as the length of calls and the numbers that are dialed.

Snowden, in a statement sent by journalist Glenn Greenwald, applauded the ruling.

"I acted on my belief that the NSA's mass surveillance programs would not withstand a constitutional challenge, and that the American public deserved a chance to see these issues determined by open courts," he said. "Today, a secret program authorized by a secret court was, when exposed to the light of day, found to violate Americans' rights. It is the first of many."

In its defense, the NSA says the data collected are key to spotting possible terrorism plots and do not include the recording of actual phone conversations. Judge Leon wrote, however, that the program likely violated Americans' right to be free of unreasonable searches.