ASHINGTON, Nov. 15 — Secretary of Defense Donald H. Rumsfeld sought today to address growing criticism over plans to try terrorists before military tribunals, saying he would move in a "very measured and conservative" way to establish procedures for such trials.

"It's not something we want to deal with on an ad hoc basis as it happens," Mr. Rumsfeld said.

In comments at the Pentagon, Mr. Rumsfeld said he had asked the department's general counsel, William J. Haynes II, to begin writing the procedures for the tribunals, which would be the first since World War II. Mr. Haynes, he said, would review precedents in American history dating to the Revolutionary War.

Mr. Rumsfeld acknowledged that the rules for military tribunals would be decidedly differently from those for civilian trials. And Pentagon officials said today that they were devising regulations that were likely to include a more flexible standard for evidence than civilian trials would accept. They said the tribunals would probably allow conviction of a suspected terrorist on a two-thirds vote of the officers on the panel.

Such departures have drawn complaints from civil liberties groups and members of Congress.

Senator Arlen Specter, Republican of Pennsylvania, took to the Senate floor today to demand hearings on the plan, contending that the White House was bypassing Congress and unilaterally expanding its powers.

Senator Patrick J. Leahy, the Vermont Democrat who is chairman of the Judiciary Committee, quickly picked up a microphone to agree with Mr. Specter.

He then added complaints about other recent administration actions taken without Congressional participation.

Mr. Leahy said tonight that he would hold hearings immediately after Thanksgiving and that he would expect Attorney General John Ashcroft to answer questions at them.

One issue that remained unclear was exactly which prisoners would qualify for a military tribunal. At the least, the tribunals would be used for people arrested overseas who could then be tried on military bases or, as Pentagon officials said today, on ships at sea.

Vice President Dick Cheney and other White House officials suggested on Wednesday that foreigners arrested in the United States might also be subject to the tribunals, but senior Justice Department officials tonight said they were unaware of any such plans.

The officials also said they had no indication that any of the more than 1,100 people taken into custody since the Sept. 11 attacks would be transferred to military custody.

Still, a senior Pentagon official said today that the first suspects could be transferred to military custody before tribunal procedures were completed, suggesting that turnovers could come at any time.

George J. Terwilliger III, a former deputy attorney general in the first Bush administration who has been an adviser to the Justice Department, said in a speech today that it might also be permissible for the United States government to transfer terrorist suspects to other nations with different standards of interrogation.

"We can't be picking and choosing our friends," Mr. Terwilliger said at a meeting of the Federalist Society, a conservative lawyers group that was holding its annual meeting in Washington.

There appeared to be widespread support for the administration's actions among those at the meeting, though some conservatives voiced rumblings of disquiet over the increasing authority of the federal government.

Prof. Eugene Volokh, who teaches at the law schools of the University of California at Los Angeles and George Mason University, said that "many people, conservatives, are feeling tension between the extension of government power and the realization that there are real security concerns at issue here."

Eugene B. Meyer, the executive director of the Federalist Society, said the expansion of federal power might divide some in the group because of the traditional dichotomy of conservatives who are libertarians and those who favor a strong governmental authority.

Manuel J. Klausner, a Los Angeles lawyer and founding editor of the libertarian magazine Reason who was attending the meeting, said that the libertarians usually made an exception to their distaste for government in the case of national security.

"I believe in appropriate defense, because this is a matter of our survival," Mr. Klausner said. But he added that he hoped the steps taken by the administration would be short-lived.

Stewart A. Baker, a Washington lawyer and former general counsel of the National Security Agency, said that he was mildly concerned about the changes but generally supported the need for new procedures.

"I don't think anyone wants to see Osama bin Laden brought before a court here to be defended by Johnnie Cochran," he said.