President George W Bush has admitted he authorised secret monitoring of communications within the United States in the wake of the 2001 terror attacks.
The monitoring was of "people with known links to al-Qaeda and related terrorist organisations", he said.
He said the programme was reviewed every 45 days, and he made clear he did not plan to halt the eavesdropping.
He also rebuked senators who blocked the renewal of his major anti-terror law, the Patriot Act, on Friday.
By preventing the extension of the act, due to expire on 31 December, they had, he said, acted irresponsibly and were endangering the lives of US citizens.
The president, who was visibly angry, also suggested that a New York Times report which had revealed the monitoring on Friday had been irresponsible.
America's enemies had "learned information they should not have", he said in his weekly radio address, which was delivered live from the White House after a pre-recorded version was scrapped.
Senators from both Mr Bush's Republican party and the opposition Democrats expressed concerns about the monitoring programme on Friday.
Senator Arlen Specter, the Republican chairman of the Senate Judiciary Committee , said there was no doubt it was "inappropriate", adding that Senate hearings would be held early next year as "a very, very high priority".
"This is Big Brother run amok," was the reaction of Democratic Senator Edward Kennedy.
Senator Russell Feingold, another Democrat, called it a "shocking revelation" that "ought to send a chill down the spine of every senator and every American".
But in his address on Saturday, Mr Bush said the programme was "critical to saving American lives".
The president said some of the 11 September hijackers inside the US had communicated with associates outside before the attacks - but the US had not known that until it was too late.
"The American people expect me to do everything in my power, under our laws and Constitution, to protect them and our civil liberties," he said.
Monitoring was, he said, a "vital tool in our war against the terrorists".
He said Congressional leaders had been briefed on the programme, which he has already renewed more than 30 times.
Mr Bush harshly criticised the leak that had made the programme public.
"Revealing classified information is illegal. It alerts our enemies," he said.
The New York Times reported on Friday that Mr Bush had signed a secret presidential order following the attacks on 11 September 2001, allowing the National Security Agency to track the international telephone calls and e-mails of hundreds of people without referral to the courts.
Previously, surveillance on American soil was generally limited to foreign embassies.
American law usually requires a secret court, known as a Foreign Intelligence Surveillance Court, to give permission before intelligence officers can conduct surveillance on US soil.