Washington ? Facing angry criticism and challenges to his authority in Congress, President Bush on Saturday unapologetically defended his administration’s right to conduct secret post-Sept. 11 spying in the United States as “critical to saving American lives.”

Bush said congressional leaders had been briefed on the operation more than a dozen times. That included Democrats as well as Republicans in the House and Senate, a GOP lawmaker said.

House Minority Leader Nancy Pelosi, D-Calif., said she had been told on several occasions that Bush had authorized unspecified activities by the National Security Agency, the nation’s largest spy agency. She said she had expressed strong concerns at the time, and that Bush’s statement Saturday “raises serious questions as to what the activities were and whether the activities were lawful.”

Often appearing angry in an eight-minute address, the president made clear he has no intention of halting his authorizations of the monitoring activities and said public disclosure of the program by the news media had endangered Americans.

Bush’s willingness to publicly acknowledge a highly classified spying program was a stunning development for a president known to dislike disclosure of even the most mundane workings of his White House. A day earlier he had refused to talk about it.

Since October 2001, the National Security Agency has eavesdropped on the international phone calls and e-mails of people inside the United States without court-approved warrants. Bush said steps like these would help fight terrorists like those involved in the Sept. 11 plot.

“The activities I have authorized make it more likely that killers like these 9-11 hijackers will be identified and located in time,” Bush said. “And the activities conducted under this authorization have helped detect and prevent possible terrorist attacks in the United States and abroad.”

Already, the administration was under fire for allegedly operating secret prisons in Eastern Europe and shipping suspected terrorists to other countries for harsh interrogations.

The NSA program’s existence surfaced as Bush was fighting to save the expiring provisions of the USA Patriot Act, the domestic anti-terrorism law enacted after the attacks of Sept. 11, 2001. Democrats and a few Republicans who say the law gives so much latitude to law enforcement officials that it threatens Americans’ constitutional liberties succeeded in stalling its renewal.