"This is a highly classified program that is crucial to our national security," he said in a radio address delivered live from the White House's Roosevelt Room.
"This authorization is a vital tool in our war against the terrorists. It is critical to saving American lives. The American people expect me to do everything in my power, under our laws and Constitution, to protect them and their civil liberties and that is exactly what I will continue to do as long as I am president of the United States," Bush said.
Angry members of Congress have demanded an explanation of the program, first revealed in Friday's New York Times. They also want to know if the monitoring by the National Security Agency violates civil liberties.
Defending the program, Bush said in his address that it is used only to intercept the international communications of people inside the United States who have been determined to have "a clear link" to al Qaeda or related terrorist organizations.
He said the program is reviewed every 45 days, using fresh threat assessments, legal reviews by the Justice Department, White House counsel and others, and information from previous activities under the program.
Without identifying specific lawmakers, Bush said congressional leaders have been briefed more than a dozen times on the program's activities.
First Amendment legal expert Floyd Abrams told CBS News correspondent Randall Pinkston that the president says that not getting congressional approval is mitigated by the fact that he had informed members of Congress.
"It is mitigated in the sense that Congress was not completely uninformed," Abrams said. "As a legal matter though, what the president has authorized 30 times may have been illegal 30 times."
The president also said the intelligence officials involved in the monitoring receive extensive training to make sure civil liberties are not violated.
Appearing angry at times during his eight-minute address, Bush left no doubt that he will continue authorizing the program.
"I intend to do so for as long as our nation faces a continuing threat from al Qaeda and related groups," he said.
Stunned Democrats quickly hit back, reports CBS News correspondent Joie Chen.
"He is President George Bush not King George Bush," Sen. Russell Feingold, D-Wis., told Chen. "He needs to back off and show respect for our system which is Congress makes the laws."
"There is no doubt that this is inappropriate," Republican Sen. Arlen Specter of Pennsylvania, chairman of the Senate Judiciary Committee, declared Friday. He promised hearings early next year.