WASHINGTON -- U.S. spy agencies have secretly collected details of billions of telephone calls made by Americans, it was revealed yesterday, setting off a political firestorm as the stunning scope of the clandestine data-gathering became clear.

In a hastily arranged, nationwide televised address aimed at defusing the political bombshell, President George W. Bush tersely insisted the government wasn't "trolling through the personal lives of millions of innocent Americans" but only hunting al-Qaeda terrorists.

Call records, not tapes of actual conversations, were amassed by the National Security Agency.

According to USA Today, which first reported the program yesterday, call details had been handed over by AT&T Inc., Verizon Communications Inc. and BellSouth Corp., three of the biggest telephone companies in the United States. The Bush administration first asked for the records soon after the terrorist attacks of Sept. 11, 2001. One major telecom company, Qwest Communications International Inc., refused and the government apparently made no effort to force compliance.

In a statement after the story broke, Mr. Bush insisted the privacy of ordinary Americans is fiercely protected." He didn't explicitly confirm or deny the USA Today report.

The first victim of unleashed congressional anger over the program could be Mr. Bush's pick to head the Central Intelligence Agency, General Michael Hayden, a former NSA chief whose nomination was already controversial because of his role as the architect of a clandestine program for eavesdropping on international phone calls from terrorist suspects to and from the United States.

"It is our government, it's not one party's government," said a visibly angry Senator Patrick Leahy, a Vermont Democrat. "Those entrusted with great power have a duty to answer to Americans for what they are doing," Other leading Democrats warned of a constitutional challenge to the embattled Bush administration.

Hunting down al-Qaeda suspects still garners widespread support among Americans, but anything that smacks of Big Brother invading personal privacy triggers fierce and negative reaction.

The Communications Security Establishment, the Canadian agency that intercepts electronic communication, is prohibited from intercepting purely domestic calls. "CSE does not have any sort of domestic-surveillance program," spokesman Adrian Simpson said.

Mohammed Nakhooda, spokesman for Bell Canada, by far the largest telephone-service provider in Canada, said the company "only provides records" to Canadian spy agencies or police when "compelled by law." But, he added: "If we are compelled by law, we are prohibited" from disclosing what we are handing over.

The details of most calls between the United States and Canada, as well as other countries, would be in the huge and growing database compiled by the NSA.

Using sophisticated algorithms and powerful supercomputers, those billions of records can be trolled by data-mining programs that flag suspicious calls or patterns or abnormal events. Once triggered, the program can then alert intelligence officers to those calls warranting further investigation.

"The NSA spying program is not only focused on terrorists or international calls," warned Anthony Romero, executive director of the American Civil Liberties Union. "The government is clearly tracking the calls and communications of millions of ordinary [people, which] serves only as further proof of how far we have slid into an abuse of power that undercuts the values Americans hold dear."

Mr. Bush reacted with unprecedented alacrity, appearing before the cameras within hours of the news breaking in an effort to allay fears about government snooping.

"Our intelligence activities strictly target al-Qaeda and their known affiliates. Al-Qaeda is our enemy, and we want to know their plans," Mr. Bush said. He also said the so-called Gang of Eight, the senior Republicans and Democrats in both the House of Representatives and the Senate, have been fully briefed about the program.

Gen. Hayden said "everything the NSA does is lawful." His confirmation hearings scheduled for next week seem certain to be the next act in an unfolding political drama as the President, already mired in a swamp of record-low approval ratings, struggles to revitalize his image as a wartime leader protecting Americans.

"The government has no interest in knowing what innocent Americans are talking about on their domestic phone calls," said Mr. Bush's spokeswoman, Dana Perino, aboard Air Force One as Mr. Bush flew west to deliver a commencement address in Biloxi, Miss. "If you are calling to make reservations at a restaurant, and if you are calling your daughter at college, or if you are calling to plan your wedding, the government has no interest."

But, echoing Mr. Bush's gunslinger talk, she added, "If al-Qaeda is planning an attack in America, you can bet that we want to make sure that we get ahead of that to prevent that and to save lives."

Former presidential contender John Kerry, a Massachusetts senator, accused the Bush administration of unjustly spying on citizens. "The NSA isn't just listening to international calls but is collecting the phone-call records of tens of millions of Americans who aren't suspected of wrongdoing," Mr. Kerry said.

The legality of compiling vast databases is uncertain, although it seems that the companies voluntarily gave the NSA proprietary corporate information, stripped of personal details such as names and addresses. But once armed with a telephone number, other data could easily be compiled.

USA Today's blockbuster front-page story yesterday included carefully worded disclaimers, but no denials, from the big telephone companies.

"We don't comment on national-security matters," Verizon was quoted as saying "We do act in full compliance with the law and we are committed to safeguarding our customers' privacy."

AT&T Inc. said: "We only assist law enforcement and government agencies charged with protecting national security in strict accordance with the law."

Data mining, a field at the frontiers of marketing, mathematics and information gathering, is still evolving, so far with few legal boundaries.

But yesterday's revelation may pit Mr. Bush's constitutional rights as commander-in-chief to wage war on all fronts against terrorist threats against one of the fundamental constitutional protections Americans hold dear.

The Fourth Amendment says: "The right of the people to be secure in their persons, houses, papers and effects, against unreasonable searches and seizures, shall not be violated."

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