George Bush said on Monday that he would continue the programme "for so long as the nation faces the continuing threat of an enemy that wants to kill American citizens" and added it included safeguards to protect civil liberties.
The president stood at a podium in the East Room of the White House, hours after a prime-time nationwide speech from the Oval Office in which he said he would prosecute the war in Iraq to a successful conclusion.
In opening news conference remarks, Bush said the spying, conducted by the National Security Agency without court oversight, was an essential element in the "war on terror".
"It was a shameful act for someone to disclose this important programme in a time of war. The fact that we're discussing this programme is discussing the enemy," he said.
The existence of the programme was disclosed last week, triggering an outpouring of criticism in Congress and an unflinching defence from Bush and senior officials of his administration.
Bush bristled when asked whether there are limits on presidential power in wartime.
"I just described limits on this particular programme, and that's what's important for the American people to understand," Bush said.
Bush: The [spying] action enables
us to move faster and quicker
Normally, no wiretapping is permitted in the US without a court warrant. But Bush said he approved the action without such orders "because it enables us to move faster and quicker. We've got to be fast on our feet".
"It is legal to do so. I swore to uphold the laws. Legal authority is derived from the constitution," he added.
Raising his voice, Bush challenged Harry Reid, the Senate Democratic leader, and Hillary Rodham Clinton, Democratic senator - without naming them - to allow a final vote on legislation renewing the anti-terrorism Patriot Act.
"I want senators from New York or Los Angeles or Las Vegas to explain why these cities are safer" without the extension, Bush said.
Reid represents Nevada; Clinton is a New York senator, and both helped block passage of the legislation in the Senate last week.
"In a war on terror, we cannot afford to be without this law for a single moment," he said.
"In a war on terror, we cannot afford to be without [the Patriot Act] for a single moment"
The legislation has cleared the House of Representatives but Senate Democrats have blocked final passage and its prospects are uncertain in the final days of the congressional session.
The president spoke not long after Attorney-General Alberto Gonzales said Congress had given Bush authority to spy on suspected terrorists in this country in legislation passed after the attacks of 11 September 2001.
Bush and other officials have said the programme involved monitoring phone calls and emails of individuals in the US believed to be plotting with terrorists overseas.
He emphasised that only international calls were monitored without court order - those originating in the US or those placed from overseas to individuals living in the US.
Bush stressed that calls placed and received within the US would be monitored as has long been the case, after an order is granted by a secret court under the provisions of the Foreign Intelligence Surveillance Act.
One of the principal provisions of the Patriot Act permitted the government to gain warrants in cases involving investigations into suspected terrorists in the US - an expansion of powers previously limited to intelligence cases.