At a press conference in Washington, Bush also called on senators to "stop their delaying tactics" and approve the Patriot Act before portions expire on Dec. 31. Civil-liberties concernson a four-year extension last week.
"Congress has a responsibility to give our law enforcement and intelligence officials the tools they need," Bush told reporters during a press conference. "We cannot afford to be without this law for a single moment."
Disclosure of the surveillance program has roiled Washington since the New York Times reported it on Friday. Republican Sen. Arlen Specter, who heads the Senate Judiciary Committee, already has pledged to convene hearings on how the program captured Americans' telephone calls, e-mail and Web browsing.
Bush and U.S. Attorney General Alberto Gonzales have said that people targeted are those suspected to have ties with al-Qaida or be working to support terrorists. "I can't get into the specific numbers because that information remains classified," Gonzalez said during a separate press briefing Monday morning.
For his part, the president responded to opposition Monday by saying the secret program would continue and warning Congress not to hold public hearings. "An open debate about law would say to the enemy, 'This is what we're going to do,'" Bush said. "Any public hearings on programs will say to the enemy, 'This is what they do--adjust.'"
At their own news conference following Bush's remarks, Senate Democrats called the surveillance program illegal. It involves using the National Security Agency to eavesdrop on international phone calls and Internet activities of people within the United States. Such eavesdropping was done without working with a secret court created by thefor this purpose.
"Where does he (Bush) find in the Constitution to tap the wires and phones of American citizens without any court oversight?" asked Sen. Carl Levin of Michigan, the senior Democrat on the Armed Services Committee.
Levin pointed out that, in an emergency, FISA permits police to conduct warrantless surveillance for 72 hours and seek retroactive approval from the secret court.
Sen. Russ Feingold, a member of the Foreign Relations Committee, said Bush's logic meant that Congress did not need to exist. "We don't have a role if the president can just make up these laws," Feingold said. "The president does not have a leg to stand on legally."