The arguments came less than two weeks after the Supreme Court ruled that the Bush administration overstepped its authority in setting up military tribunals to try war on terror detainees held at a US naval base in Guantanamo Bay, Cuba.

The American Civil Liberties Union asked a judge in Detroit, Michigan to rule that the wiretaps are illegal because they circumvent the Foreign Intelligence Surveillance Act, which requires the executive branch to obtain a warrant before engaging in electronic surveillance of Americans.

Those warrants are obtained in a secret court which has only denied three requests in nearly 30 years and which allows law enforcement to initiate surveillance before the warrant is obtained, Michael Steinberg, legal director of the ACLU of Michigan, told AFP.

"Our democracy depends on checks and balances," he said. "Obtaining a court order before intercepting people's personal communication serves as a check on unbridled executive power."

A spokesman for the Department of Justice declined to comment on the case.

In a motion, the department argued that "The United States submits that the actions authorized by the President are essential to meeting a continuing and grave foreign terrorist threat and are well within lawful bounds."

It argued that to demonstrate this "would require evidence that must be excluded from consideration under the military and states secrets privilege" and as a result the judge should throw out the case in order to avoid "grave harm to the national security of the United States."

The ACLU argued that the existence of the program was far from a secret: it has already acknowledged and defended the spying program to the public and issued a 40-page paper discussing in detail its legal defenses and justifications in January.

"If the court accepts the government's state secrets argument we would be facing a constitutional crisis because the government is essentially arguing it can ignore the law and is thumbing its nose at the constitution and the courts can't do anything about it because it's a secret program," Steinberg said. "There's no conceivable end to this argument."

Steinberg said the ACLU's case was strengthened by the recent ruling over the Guantanamo tribunals, which reaffirmed that the president does not have unlimited authority to act without Congressional approval during its "war on terror."

A ruling is expected in the coming weeks.

The ACLU is also filing suit against the telephone companies which participated in the program in violation of their obligations to customers.