WASHINGTON (AFP) - A US judge struck a first blow against he National Security Agency's (NSA) bulk collection of phone records Monday, ruling it breaches citizens' privacy to a degree that is probably unconstitutional.
The ruling by the Washington District Court was stayed pending appeal, but if upheld it would bar the spy agency from indiscriminately gathering metadata on millions of private calls.
"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," Judge Richard Leon said.
Leon argued that American founding father James Madison, one of the authors of the US Constitution, would be "aghast" at the government's breach of citizens' rights to privacy.
Two plaintiffs, Mr Larry Klayman and Mr Charles Strange, brought a case against President Barack Obama's administration after leaker Edward Snowden revealed the scope of the NSA snooping.
Documents provided by Snowden to the British daily The Guardian revealed that the NSA had compelled US telecoms giant Verizon to provide agents with its clients' call metadata.
Subsequent leaks from Snowden's vast trove of classified documents suggest that other telephone and internet providers have also provided private details on both US and foreign clients.
Judge Leon's ruling would bar the US government from continuing to siphon off metadata without judicial approval, but he denied a request for an immediate injunction against the practice.
Instead the case will go to appeal.
Mr Obama's administration has condemned Snowden's leaks and insists that collecting metadata - the time, duration and destination of calls but not their content - is not unconstitutional.
US spy chiefs say that by joining the dots between archived calls and terrorist suspects they can keep America safe, but the Verizon case will not be the last legal challenge the programme faces.
On Friday, an official panel handed Mr Obama a secret review of the NSA's electronic surveillance program along with more than 40 recommendations to install safeguards and limit its scope.
But the administration is not expected to significantly curtail the NSA's mission, despite international outrage, and Snowden remains a fugitive from US justice granted temporary asylum in Russia.
Judge Leon said the appeal process would take at least six months and delayed his order "in light of the significant national security interests at stake ... and the novelty of the constitutional issues."