By Adam Brookes
BBC News, Washington
The Bush administration has embarked on a concerted public defence of its intelligence-gathering practices.
The Bush administration says the eavesdropping is necessary
It is focusing particularly on the electronic surveillance of Americans suspected of links to international terrorism.
Attorney General Alberto Gonzales said eavesdropping on citizens' international calls without court approval was legal and necessary.
President Bush is also expected to address this issue on Wednesday.
The news that US intelligence agencies have been listening to the international communications of Americans without getting the usual permission from the courts to do so is creating some political turbulence for the Bush administration.
Democrats in Congress, and some Republicans, have expressed anger at what they see as an infringement of civil liberties, and Congressional researchers have even debated the programme's legality.
Campaign of persuasion
So this week the administration is going all out to persuade Americans that the surveillance is legal and has been effective in protecting the country from further attacks like those of 11 September 2001.
Alberto Gonzales, who heads the Department of Justice, said in a speech at Washington's Georgetown University that Congress had given the president authority to order the surveillance without going through the courts.
He said that the normal procedures for getting warrants to run surveillance were too slow and cumbersome.
On Monday one of the country's most senior intelligence officers, General Michael Hayden, made a rare public appearance and a similar argument.
This is a very concerted campaign of persuasion and it suggests that the administration is rattled by allegations that it might have broken the law.
A new opinion poll suggests that a distinct majority of Americans, some 58%, favour a full investigation into whether or not the surveillance is legal.