"This is a highly classified programme that is crucial to our national security," President Bush said in a radio address delivered live from the White House on Saturday.
"This authorisation 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."
Members of Congress have demanded an explanation of the programme, revealed in Friday's New York Times, and want to know whether the monitoring by the National Security Agency without obtaining warrants from a court violates civil liberties.
Bush said the programme was narrowly designed and used "consistent with US law and the constitution". He said it was used only to intercept the international communications of people inside the United States who had been determined to have "a clear link" to al-Qaida or related terrorist organisations.
The programme is reviewed every 45 days, using current threat assessments, legal reviews by the Justice Department, White House counsel and others, and information from previous activities under the programme, the president said.
Without identifying specific politicians, Bush said congressional leaders had been briefed more than a dozen times on the programme's activities.
"He's President George Bush, not King George Bush. This is not the system of government we have and that we fought for"
The president also said the intelligence officials involved in the monitoring received extensive training to make sure that civil liberties were not violated.
Appearing angry at points during his eight-minute address, Bush said he had re-authorised the programme more than 30 times since the 11 September attacks and planned to continue doing so.
"I intend to do so for as long as our nation faces a continuing threat from al-Qaida and related groups," he said.
Bush's remarks echoed those issued on Friday night by a senior intelligence official who spoke on condition of anonymity. His unusual discussion of classified activities showed the sensitive nature of the programme, whose existence was revealed as Congress was trying to renew the Patriot Act.
Reacting to Bush's defence of the NSA programme, Russell Feingold, a Democrat senator, said the president's remarks were "breathtaking in how extreme they were".
Feingold said it was "absurd" that Bush said he relied on his inherent power as president to authorise the wiretaps.
"If that's true, he doesn't need the Patriot Act because he can just make it up as he goes along," he said.
"I tell you, he's President George Bush, not King George Bush. This is not the system of government we have and that we fought for."