The super-secretive NSA, which has generally been barred from domestic spying except in narrow circumstances involving foreign nationals, has monitored the e-mail, telephone calls and other communications of hundreds, and perhaps thousands, of people under the program, the New York Times disclosed last night.

The aim of the program was to rapidly monitor the phone calls and other communications of people in the United States believed to have contact with suspected associates of al Qaeda and other terrorist groups overseas, according to two former senior administration officials. Authorities, including a former NSA director, Gen. Michael V. Hayden, were worried that vital information could be lost in the time it took to secure a warrant from a special surveillance court, sources said.

But the program's ramifications also prompted concerns from some quarters, including Sen. John D. Rockefeller IV (W.Va.), the ranking Democrat on the intelligence committee, and the presiding judge of the surveillance court, which oversees lawful domestic spying, according to the Times.

The Times said it held off on publishing its story about the NSA program for a year after administration officials said its disclosure would harm national security.

The White House made no comment last night. A senior official reached by telephone said the issue was too sensitive to talk about. None of several press officers responded to telephone or e-mail messages.

Congressional sources familiar with limited aspects of the program would not discuss any classified details but made it clear there were serious questions about the legality of the NSA actions. The sources, who demanded anonymity, said there were conditions under which it would be possible to gather and retain information on Americans if the surveillance were part of an investigation into foreign intelligence.

But those cases are supposed to be minimized. The sources said the actual work of the NSA is so closely held that it is difficult to determine whether it is acting within the law.

The revelations come amid a fierce congressional debate over reauthorization of the USA Patriot Act, an anti-terrorism law passed after the Sept. 11, 2001, attacks. The Patriot Act granted the FBI new powers to conduct secret searches and surveillance in the United States.

Most of the powers covered under that law are overseen by a secret court that meets at Justice Department headquarters and must approve applications for wiretaps, searches and other operations. The NSA's operation is outside that court's purview, and according to the Times report, the Justice Department may have sought to limit how much that court was made aware of NSA activities.

Public disclosure of the NSA program also comes at a time of mounting concerns about civil liberties over the domestic intelligence operations of the U.S. military, which have also expanded dramatically after the Sept. 11 attacks.

For more than four years, the NSA tasked other military intelligence agencies to assist its broad-based surveillance effort directed at people inside the country suspected of having terrorist connections, even before Bush signed the 2002 order that authorized the NSA program, according to an informed U.S. official.

The effort, which began within days after the attacks, has consisted partly of monitoring domestic telephone conversations, e-mail and even fax communications of individuals identified by the NSA as having some connection to al Qaeda events or figures, or to potential terrorism-related activities in the United States, the official said.

It has also involved teams of Defense Intelligence Agency personnel stationed in major U.S. cities conducting the type of surveillance typically performed by the FBI: monitoring the movements and activities -- through high-tech equipment -- of individuals and vehicles, the official said.

The involvement of military personnel in such tasks was provoked by grave anxiety among senior intelligence officials after the 2001 suicide attacks that additional terrorist cells were present within U.S. borders and could only be discovered with the military's help, said the official, who had direct knowledge of the events.

Kate Martin, director of the Center for National Security Studies, said the secret order may amount to the president authorizing criminal activity.

The law governing clandestine surveillance in the United States, the Foreign Intelligence Surveillance Act, prohibits conducting electronic surveillance not authorized by statute. A government agent can try to avoid prosecution if he can show he was "engaged in the course of his official duties and the electronic surveillance was authorized by and conducted pursuant to a search warrant or court order of a court of competent jurisdiction," according to the law.

"This is as shocking a revelation as we have ever seen from the Bush administration," said Martin, who has been sharply critical of the administration's surveillance and detention policies. "It is, I believe, the first time a president has authorized government agencies to violate a specific criminal prohibition and eavesdrop on Americans."

Caroline Fredrickson, director of the Washington legislative office of the American Civil Liberties Union, said she is "dismayed" by the report.

"It's clear that the administration has been very willing to sacrifice civil liberties in its effort to exercise its authority on terrorism, to the extent that it authorizes criminal activity," Fredrickson said.

The NSA activities were justified by a classified Justice Department legal opinion authored by John C. Yoo, a former deputy in the Office of Legal Counsel who argued that congressional approval of the war on al Qaeda gave broad authority to the president, according to the Times.

That legal argument was similar to another 2002 memo authored primarily by Yoo, which outlined an extremely narrow definition of torture. That opinion, which was signed by another Justice official, was formally disavowed after it was disclosed by the Washington Post.

Justice Department spokeswoman Tasia Scolinos would not comment on the report last night.

Staff writers Dafna Linzer and Peter Baker contributed to this report.