The White House has sought to justify its surveillance of millions of Americans' phone records as anger grows over revelations that a secret court order gives the National Security Agency blanket authority to collect call data from a major phone carrier.

Politicians and civil liberties campaigners described the disclosures, revealed by the Guardian on Wednesday, as the most sweeping intrusion into private data they had ever seen by the US government.

But the Obama administration, while declining to comment on the specific order, said the practice was "a critical tool in protecting the nation from terrorist threats to the United States".

The secret Foreign Intelligence Surveillance Court (Fisa) granted the order to the FBI on April 25, giving the government unlimited authority to obtain the data for a specified three-month period ending on July 19.

Under the terms of the blanket order, the numbers of both parties on a call are handed over, as is location data, call duration, unique identifiers, and the time and duration of all calls. The contents of the conversation itself are not covered.

The disclosure has reignited longstanding debates in the US over the proper extent of the government's domestic spying powers.

Under the Bush administration, officials in security agencies had disclosed to reporters the large-scale collection of call records data by the NSA, but this is the first time significant and top-secret documents have revealed the continuation of the practice under President Obama.

The White House stressed that orders such as the one disclosed by the Guardian would only cover data about the calls rather than their content. A senior administration official said: "Information of the sort described in the Guardian article has been a critical tool in protecting the nation from terrorist threats to the United States, as it allows counter-terrorism personnel to discover whether known or suspected terrorists have been in contact with other persons who may be engaged in terrorist activities, particularly people located inside the United States.

"As we have publicly stated before, all three branches of government are involved in reviewing and authorising intelligence collection under the Foreign Intelligence Surveillance Act. Congress passed that act and is regularly and fully briefed on how it is used, and the Foreign Intelligence Surveillance Court authorises such collection. There is a robust legal regime in place governing all activities conducted pursuant to the Foreign Intelligence Surveillance Act."

The administration stressed that the court order obtained by the Guardian relates to call data, and does not allow the government to listen in to anyone's calls.

However, in 2013, such metadata can provide authorities with vast knowledge about a caller's identity. Particularly when cross-checked against other public records, the metadata can reveal someone's name, address, driver's licence, credit history, social security number and more. Government analysts would be able to work out whether the relationship between two people was ongoing, occasional or a one-off.

"From a civil liberties perspective, the program could hardly be any more alarming. It's a program in which some untold number of innocent people have been put under the constant surveillance of government agents," said Jameel Jaffer, American Civil Liberties Union deputy legal director. "It is beyond Orwellian, and it provides further evidence of the extent to which basic democratic rights are being surrendered in secret to the demands of unaccountable intelligence agencies."

The order names Verizon Business Services, a division of Verizon Communications. In its first-quarter earnings report, published in April, Verizon Communications listed about 10 million commercial lines out of a total of 121 million customers. The court order does not specify what type of lines are being tracked. It is not clear whether any additional orders exist to cover Verizon's wireless and residential customers, or those of other phone carriers.

Fisa court orders typically direct the production of records pertaining to a specific, named target suspected of being an agent of a terrorist group or foreign state, or a finite set of individually named targets. The unlimited nature of the records being handed over to the NSA is extremely unusual.

The Verizon order expressly bars the company from disclosing to the public either the existence of the FBI's request for its customers' records, or the court order itself. "We decline comment," said Ed McFadden, a Washington-based Verizon spokesman said on Wednesday.

News of the order brought swift condemnation from senior US politicians. Former vice-president Al Gore described the "secret blanket surveillance" as "obscenely outrageous". "In [the] digital era, privacy must be a priority," he said.

The court order appears to explain the numerous cryptic public warnings by two US senators, Mark Udall and Ron Wyden, about the scope of the Obama administration's surveillance activities.

For about two years, the two Democrats have been stridently advising the public that the US government is relying on "secret legal interpretations" to claim surveillance powers so broad that the American public would be "stunned" to learn of the kind of domestic spying being conducted.

Udall, a member of the Senate intelligence committee, said on Wednesday night: "While I cannot corroborate the details of this particular report, this sort of widescale surveillance should concern all of us and is the kind of government overreach I've said Americans would find shocking."

The Center for Constitutional Rights said in a statement that the secret court order was unprecedented. "As far as we know this order from the Fisa court is the broadest surveillance order to ever have been issued: it requires no level of suspicion and applies to all Verizon [business services] subscribers anywhere in the US.

"The Patriot Act's incredibly broad surveillance provision purportedly authorizes an order of this sort, though its constitutionality is in question and several senators have complained about it."

Mark Rumold, a lawyer with the Electronic Frontier Foundation, said: "This is confirmation of what we've long feared, that the NSA has been tracking the calling patterns of the entire country. We hope more than anything else that the government will allow a judge to decide whether this is constitutional, and we can finally put an end to this practice."

Howard Wolfson, a deputy mayor of New York, described the revelations as "a shocking report that really exploded overnight".

"A lot of people are waking up now and I think they will be horrified," he said. "It is not just the civil libertarian wings of the Republican and Democratic parties; I think most Americans will be really surprised that their government is having access to all of the phone calls they make."

"I don't think the administration's response [so far] is anywhere near adequate. I think you will see a lot of questions being asked in the coming days."

Oregon senator Jeff Merkley said: "This type of secret bulk data collection is an outrageous breach of Americans' privacy. Can the FBI or the NSA really claim that they need data scooped up on tens of millions of Americans?"