Eavesdropping on terrorists and their associates in America is a battlefield prerogative that belongs to President Bush and cannot be taken away, a Justice Department attorney told a federal judge in Manhattan yesterday.

The statement came during a hearing in a lawsuit that challenges the Bush administration’s terrorist surveillance program on behalf of several attorneys at the Manhattan-based Center for Constitutional Rights.

The Justice Department attorney, Anthony Coppolino, said the power to conduct war time surveillance has long been exercised by presidents and “derives from the Constitution.”

Since the New York Times first reported the existence of the surveillance program in December 2005, administration officials have confirmed that the eavesdropping occurs without a court warrant, and have said it targets communications between persons on American soil and Al Qaeda associates abroad.

“This is an area that is as close to the modern-day battlefield as you can get,” Mr. Coppolino said in U.S. District Court in Manhattan. “Where is Al Qaeda today and what are they planning to do? Where are they going to hit us?”

He argued that the Foreign Intelligence Surveillance Act — which requires the president to get a court warrant to eavesdrop in certain scenarios — intrudes on the president’s inherent constitutional powers if the law is interpreted to outlaw Mr. Bush’s program.

Almost three weeks ago, a federal judge in Detroit, Anna Diggs Taylor, ruled that the eavesdropping violated both the Fourth Amendment and FISA, and must be halted.

The ruling, which the government is appealing, was in response to a similar lawsuit by the American Civil Liberties Union. While the judge in the New York case, Gerard Lynch, is free to consider Judge Taylor’s ruling, it is not binding.

Yesterday, Judge Lynch directed few questions toward the Fourth Amendment issues. Instead, during an oral argument that lasted nearly three hours, Judge Lynch appeared skeptical of the government’s claim that Congress, when it passed FISA in 1978, had improperly taken away powers that belonged to the president.

“We’re debating a rather abstract but rather vital issue: Does the president have the power to do something despite the fact that Congress has said ‘Thou shalt not have this power’?” Judge Lynch said.

Although he said he had “no idea at this point” how he will rule, Judge Lynch questioned whether the office of the presidency came with as much inherent power as Mr. Coppolino claimed.

“Doesn’t Congress always have the power to give the president tools?” Judge Lynch said. “They can raise armies or not. They can give him the tools or not. When you say the president must have the tools, isn’t that too much?”

At one point, Judge Lynch said that what both sides demanded he do — either strike down FISA or strike down the surveillance program — amounts to judicial activism.

Judge Lynch also seemed troubled that the eavesdropping would pick up conversations in which one party was on American soil.

“Even Julius Caesar didn’t get to bring his armies back into Rome — although he did,” Judge Lynch said, referring to the Roman prohibition barring its generals from leading armies back toward Rome.

Judge Lynch began yesterday’s hearing by saying that he had not yet read the government’s secret brief on the specifics of the surveillance program.

“This is a public argument and I did not want to risk inadvertent disclosure” of a classified piece of information, Judge Lynch, who once served as a prosecutor in the Iran/Contra investigation and was later appointed to the bench by President Clinton, said. “I want to emphasize that I am so far as ignorant as the plaintiff or the public about what is in the government’s classified submissions.

The Justice Department has sought to have the suit dismissed on the grounds that litigation would require divulging secrets about the scope of the program.

Judge Lynch directed nearly all his questioning on the legality of the program to the government. He sought input from the plaintiffs for the most part during a lengthy argument on whether the plaintiffs have the standing to bring the lawsuit. Mr. Coppolino said the plaintiffs have to be able to prove that the government spied on their communications with foreign clients before the lawsuit could go forward.

A lawyer for the plaintiffs, Shayana Kadidal, said the lawyers have represented terror suspects who “are alleged to be exactly the types of people” the surveillance would target.

In briefs, the lawyers have said they have already been harmed by the existence of the surveillance program because their clients are afraid to speak openly with them and they can no longer use phone calls or e-mails to discuss their cases.