Former US vice-president Al Gore has described the activities of the National Security Agency as "outrageous" and "completely unacceptable" and said whistleblower Edward Snowden has "revealed evidence" of crimes against the US constitution.

Gore, speaking Tuesday night at McGill University in Montreal, said he was in favour of using surveillance to ensure national security, but Snowden's revelations showed that those measures had gone too far.

"I say that as someone who was a member of the National Security Council working in the White House and getting daily briefings from the CIA," Gore said, in comments reported by the Canadian Press.

Gore had previously said he believed the practice of the NSA collecting US citizens phone records was unlawful and "not really the American way", but his comments on Tuesday represent his strongest criticism yet.

Asked about Snowden, the NSA whistleblower whose revelations have been reported extensively by the Guardian, Gore said the leaks had revealed uncovered unconstitutional practices.

"He has revealed evidence of what appears to be crimes against the Constitution of the United States," Gore said.

Snowden faces criminal charges for leaking classified information to The Guardian and other media outlets. He remains in exile in Russia.

Gore, the former vice-president, 2000 Democratic presidential nominee and 2007 Nobel Peace Prize winner, said the NSA's efforts to monitor communications had gone to "absurd" lengths, the Canadian Press reported.

"When you are looking for a needle in a haystack, it's not always wise to pile more hay on the haystack," he said.

Gore said he doubted the far-reaching scope of the NSA's surveillance would be allowed to continue.

"I think they will have to pull this back," he said. "I think you will see a reining in."