WASHINGTON Two leading civil rights groups filed lawsuits Tuesday against the Bush administration over its domestic spying program to determine whether the operation had been used to monitor 10 defense lawyers, journalists, scholars, political activists and other Americans with ties to the Middle East.

The two lawsuits, which were filed separately by the American Civil Liberties Union in U.S. District Court in Detroit and by the Center for Constitutional Rights in U.S. District Court in Manhattan, are the first major court challenges to the eavesdropping program. Both groups are seeking to have the courts order an immediate end to the program, which they say is illegal and unconstitutional.

The Bush administration has strongly defended the surveillance program as legal and necessary, and officials said the Justice Department would probably vigorously oppose the lawsuits on national security grounds.

Justice Department officials, speaking before the suits were filed Tuesday, would not comment on any individuals who might have been singled out under the National Security Agency program, and they said the department would review the lawsuits once they were filed.

The lawsuits seek to answer one of the major questions surrounding the eavesdropping program: Has it been used solely to monitor the international phone calls and e-mail messages of people with known links to Al Qaeda, as President George W. Bush and his most senior advisers have maintained, or has it been abused in ways resembling the political spying abuses of the 1960s and '70s?

"There's almost a feeling of déjà vu with this program," said James Bamford, an author and journalist who is one of five individual plaintiffs in the civil liberties union's lawsuit who say they suspect the program may have been used to monitor their international communications.

"It's a return to the bad old days of the NSA," said Bamford, who has written two widely cited books on the intelligence agency.

Although the program's public disclosure last month has generated speculation that it might have been used to monitor journalists or politicians, no evidence has emerged to support that idea. Bush administration officials point to a secret audit by the Justice Department last year that reviewed a sampling of security agency interceptions involving Americans and that they said found no documented abuses.

The lawsuit to be filed by the Center for Constitutional Rights has as plaintiffs four lawyers and a legal assistant at the center who work on terrorism-related cases at Guantánamo Bay, Cuba, and overseas that often involve international e-mail messages and phone calls. Similarly, the plaintiffs in the civil liberties union's lawsuit include five Americans who work on international policy and terrorism, along with the union and three other advocacy groups.

"We don't have any direct evidence" that the plaintiffs were monitored by the security agency, said Ann Beeson, associate legal director for the civil liberties union.

"But the plaintiffs have a well-founded belief that they may have been monitored," Beeson said, "and there's a real chilling effect in the fear that they can no longer have confidential discussions with clients or sources without the possibility that the NSA is listening."

One of the civil liberties union plaintiffs, Larry Diamond, a senior fellow at the Hoover Institution, a California research institute, said that a Stanford University student in Egypt conducts research for him on political opposition groups. "How can we communicate effectively if you risk being intercepted by the National Security Agency?" Diamond asked.

Also named as plaintiffs in the ACLU lawsuit are the journalist Christopher Hitchens, who has written in support of the wars in Iraq and Afghanistan; Barnett Rubin, a scholar at New York University who works in international relations; Tara McKelvey, a senior editor at American Prospect magazine; the National Association of Criminal Defense Lawyers; Greenpeace, the environmental advocacy group; and the Council on American-Islamic Relations, the country's largest Islamic advocacy group.