The Center for Constitutional Rights plans to sue on behalf of four lawyers at the center and a legal assistant there who work on terrorism-related cases at Guantánamo Bay, Cuba, and overseas, which often involves international e-mail messages and phone calls. Similarly, the plaintiffs in the A.C.L.U. lawsuit include five Americans who work in international policy and terrorism, along with the A.C.L.U. and three other 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 A.C.L.U. "But the plaintiffs have a well-founded belief that they may have been monitored, 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 N.S.A. is listening."

One of the A.C.L.U. plaintiffs, Larry Diamond, a senior fellow at the Hoover Institute, said that a Stanford student studying in Egypt conducted research for him on political opposition groups, and that he worried that communications between them on sensitive political topics could be monitored. "How can we communicate effectively if you risk being intercepted by the National Security Agency?" Mr. Diamond said.

Also named as plaintiffs in the A.C.L.U. lawsuit are the journalist Christopher Hitchens, who has written in support of the wars in Iraq and Afghanistan; Barnett R. Rubin, a scholar at New York University who works in international relations; Tara McKelvey, a senior editor at The American Prospect; 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.

The lawsuits over the eavesdropping program come as several defense lawyers in terrorism cases have begun challenges, arguing that the government may have improperly hidden the use of the surveillance program from the courts in investigating terrorism leads.

Bill Goodman, legal director for the Center for Constitutional Rights, said that in suing in federal court to block the surveillance program, his group believed "without question" that Mr. Bush violated the Foreign Intelligence Surveillance Act, which governs wiretaps, by authorizing the security agency operation.

But Mr. Goodman acknowledged that in persuading a federal judge to intervene, "politically, it's a difficult case to make."