The Bush administration’s practice of using the National Security Agency to eavesdrop on international phone calls and read e-mails of American citizens is facing its first serious legal challenges after civil liberties groups filed separate broad federal lawsuits against the warrantless surveillance program yesterday.

The more comprehensive suit was filed in Detroit by the American Civil Liberties Union on behalf of a variety of organizations, authors, and scholars who have regular contact with affiliates and individuals abroad. The case in New York was brought by a legal advocacy group that has represented terrorism suspects, the Center for Constitutional Rights.

The executive director of the ACLU, Anthony Romero, said the legal action was needed to rein in President Bush and other officials who were operating without legal authority.

“This historic lawsuit responds to an unprecedented seizure of power of the executive branch,” Mr. Romero told reporters on a conference call yesterday. “The ACLU believes that the National Security Agency’s spying program is both unconstitutional and illegal.”

Asked about the court challenges yesterday, the White House press secretary, Scott McClellan, said, “I think that the frivolous lawsuits that you reference do nothing to help enhance civil liberties or protect the American people.”

Mr. McClellan repeated the administration’s claim that the surveillance program, which was instituted after the September 11, 2001, strikes on America, has thwarted terrorist activity.

“This authorization has been successful to prevent attacks from happening,” Mr. McClellan said. “It is absolutely a vital tool in the war on terrorism.”

A former Justice Department official who has defended Mr. Bush’s right to conduct electronic surveillance without court approval, John Schmidt, said yesterday that he does not believe the litigation will succeed. “I think it just gets dismissed,” said Mr. Schmidt, who served as associate attorney general from 1994 to 1997, under President Clinton. “It sounds to me like more of a p.r. effort than a serious lawsuit. I can’t believe that they could have standing to go in, in the abstract, and challenge the program.”

Mr. Schmidt said the legal principle of standing will require that the plaintiffs demonstrate that they have been targeted by the surveillance program. None of them are likely to be able to do so, he said. “I don’t think they’re going to be able to make a case just by asserting some probability they’ve been picked up.”

The plaintiffs in the ACLU suit include a Muslim group, the Council on American-Islamic Relations; an international environmental group, Greenpeace; the National Association of Criminal Defense Lawyers; a New York University scholar who is a leading authority on Afghanistan, Barnett Rubin, and a former writer for the Nation magazine who is a staunch supporter of the war in Iraq, Christopher Hitchens.

Bush Administration officials have said the surveillance program, which was a closely held secret until it was disclosed in a New York Times report in December, was designed to track contacts between known Al Qaeda members and their associates. However, the executive director of Green peace, John Passacantando, said his group’s activities made him fear that it was targeted by the surveillance. “The way you get caught up in this is through international e-mail, international phones calls, and international money transfers. We do all those things,” he said, adding that the lack of confidentiality could intimidate some sources of information.

Asked how he could be sure other countries were not also listening to the same calls and reading the same emails, the Greenpeace official said, “The fact that somebody else may also be able to surveil those conversations is not really the point to me. The point is that the greatest democracy on earth is in contravention of the law designed to protect their citizenry.”

Critics of the program have argued the president lost the authority to conduct warrantless surveillance domestically after the passage of the Foreign Intelligence Surveillance Act in 1978. The White House contends that the surveillance is legal because of Mr. Bush’s power under the Constitution and a 2001 congressional resolution authorizing the use of force against Al Qaeda.

Mr. Hitchens said he believes that the president and the intelligence establishment are using the war as a vehicle to expand their power beyond what is justified. “I’m fed up with it,” he told The New York Sun. “If the president doesn’t care for the law the way it is or wants to apply it in a different way, he’s obliged to seek permission to do so,” the writer said.

Mr. Hitchens said his action should not be read as an indication that his support for military action in Iraq is wavering. “I’m more pro-the-war than I ever was,” he said. “If it’s true that we’re at war, which we sure are, it makes it more important we know where the boundaries are, not less.”

Mr. Hitchens stressed that, beyond the lawsuit, he has no common cause with some of the other plaintiffs enlisted by the ACLU, in particular the Council on American-Islamic Relations. “I was revolted to see who I was in company with,” he said. “CAIR is a lot to swallow. I think they’re a shady and disgusting organization.”

A lawyer for the group, Arsalan Iftikhar, did not respond to a message yesterday seeking comment for this story.

The ACLU brought its lawsuit in Michigan because it was the site, in 1971, of one of the first definitive court rulings against warrantless surveillance of Americans. The case involved extrajudicial wiretapping of a radical group, the White Panther Party, which was accused of bombing CIA offices in Ann Arbor, Mich.

Some in the legal establishment have already indicated they have similar doubts about the Bush administration’s warrantless surveillance program. Last month, a judge who sat on the court that issues Foreign Intelligence Surveillance Act warrants, James Robertson, resigned his position. He was reportedly angry over the White House’s decision to cut the court out of the process for approving some international eavesdropping on Americans.

The lawsuit filed yesterday in Detroit was assigned to Judge Anna Taylor, an appointee of President Carter. The suit filed in New York will be heard by Judge Gerald Lynch, who was appointed by President Clinton.

Attorneys for some criminal defendants have already filed court motions seeking to determine whether the warrantless surveillance may have played a role the prosecutions of their clients. If such links are established, those criminal cases could open a second front of legal attack on the surveillance program.