At the same time, the Justice Department has asked law enforcement authorities across the country to pick up and question 5,000 men, most from Middle Eastern countries, who entered the country legally in the last two years.

Both actions are part of a sweeping government effort to expand the investigation into Al Qaeda's network and clear the way for the more aggressive prosecution of anyone charged with terrorism.

Mr. Bush signed the order allowing for the military tribunals shortly before leaving this afternoon for his ranch in Crawford, Tex. White House officials said the order did not create a military tribunal or a list of terrorists to be tried. Instead, they said, it was an ''option'' that the president would have should Osama bin Laden or his associates in Al Qaeda be captured. If the tribunals were created, it would be the first time since World War II that such an approach was used, officials said.

Under the order, the president himself is to determine who is an accused terrorist and therefore subject to trial by the tribunal. The order states that the president may ''determine from time to time in writing that there is reason to believe'' that an individual is a member of Al Qaeda, has engaged in acts of international terrorism or has ''knowingly harbored'' a terrorist.

In order to make such a finding, the president needs information, and obtaining information about Al Qaeda and the Sept. 11 terrorist acts is the goal of the Justice Department's effort to find and interview the 5,000 men, department officials said.

The people being sought are not believed to be terrorism suspects, and they will not be placed under arrest, the officials said. The interviews are intended to be voluntary.

Nonetheless, officials at the American Civil Liberties Union condemned the Justice Department effort, as well as the executive order allowing military tribunals.

Steven Shapiro, the national legal director of the A.C.L.U., called the effort to interview the 5,000 men a ''dragnet approach that is likely to magnify concerns of racial and ethnic profiling.''

Laura W. Murphy, the director of the A.C.L.U. Washington National Office, described the order regarding tribunals as ''deeply disturbing and further evidence that the administration is totally unwilling to abide by the checks and balances that are so central to our democracy.''

White House officials said the tribunals were necessary to protect potential American jurors from the danger of passing judgment on accused terrorists. They also said the tribunals would prevent the disclosure of government intelligence methods, which normally would be public in civilian courts.

''We have looked at this war very unconventionally,'' said Dan Bartlett, the White House communications director, ''and the conventional way of bringing people to justice doesn't apply to these times.''

The idea of using tribunals has been suggested by some lawyers outside the government as well.

''It's the most pragmatic way and it's the most legally correct way to adjudicate terrorist war crimes,'' said Spencer J. Crona, a Denver probate lawyer and the co-author of a 1996 article in the Oklahoma City University Law Journal arguing the merits of military tribunals to try terrorists.

Mr. Crona and his co-author, Neal A. Richardson, a deputy district attorney in Denver, have continued to promote the idea, most recently in an opinion article in September in The Los Angeles Times. Mr. Crona added that terrorists are not ''mere criminals'' but enemy agents engaged in war crimes against Americans.

But experts in military law said the tribunals would severely limit the rights of any defendant even beyond those in military trials. The tribunals, they said, did not provide for proof of guilt beyond a reasonable doubt and would not require strict rules of evidence like those in military and civilian courts.

''The accused in such a court would have dramatically fewer rights than a person would in a court-martial,'' said Eugene R. Fidell, the president of the National Institute of Military Justice.

Mr. Fidell said he expected the order to be challenged in court, adding, ''It establishes a court that departs in important respects from core aspects of American criminal justice.''

Mindy Tucker, the Justice Department spokeswoman, said tribunals would not ''preclude any Justice Department options'' but would be an ''additional tool.''

''These are obviously extraordinary times and the president needs to have as many options as possible,'' Ms. Tucker said.

In signing the military order, a highly unusual act by a president, Mr. Bush invoked his constitutional authority as commander in chief as well as the resolution authorizing military force passed by Congress on Sept. 15. Congress has not passed a formal declaration of war, and military law experts said one was not necessary for Mr. Bush's order.

White House officials said that there was precedent for the military tribunals and that they had been approved by the Supreme Court, first in 1801. Those accused of plotting the assassination of Abraham Lincoln were also tried and convicted by a military court, Bush administration officials said.

President Franklin D. Roosevelt, White House officials said, had German saboteurs tried by a military court in World War II; six of them were executed. The Supreme Court upheld the proceeding, saying that people who entered the United States to wage war were combatants who could be tried in a military court.

''What would you do if you caught bin Laden?'' one administration official said tonight. ''This is an additional option that is being provided by this order.''

Administration officials said a long, public trial might turn Mr. bin Laden into a martyr, and could cause further terrorism in his name.

The names of the 5,000 people that the Justice Department wants to interview were compiled from immigration and State Department records of people who entered the United States since Jan. 1, 2000, on tourist, student or business visas. Only men aged 18 to 33 with these visas who are living in the United States are on the list.

The names of the countries whose citizens have been placed on the list were not made public, but most are Middle Eastern nations thought to have harbored followers of Mr. bin Laden or to have been used by Al Qaeda as a staging base for activities in the United States.

Ms. Tucker, the Justice Department spokeswoman, said she hoped some of the men would help the government thwart further attacks.

The Council on American-Islamic Relations, an Islamic advocacy group based in Washington, expressed concern about the plan and said the government should publish guidelines for these interviews, including the right of those being interviewed to have legal representation.

''This type of sweeping investigation carries with it the potential to create the impression that interviewees are being singled out because of their race, ethnicity or religion,'' said Nihad Awad, the group's executive director.

On Capitol Hill, the issue of who is entering the country illegally was in the forefront today, with senators sharply questioning a senior official of the Immigration and Naturalization Service who acknowledged that immigration agents were not required to conduct criminal background checks on immigrants caught crossing the border illegally.

The official, Michael A. Pearson, the executive associate commissioner for field operations, said agents could use their discretion as to whether such a person should be detained or let go pending further action.

Mr. Pearson said that 12,338 undocumented immigrants were arrested for illegal entry along the nation's northern border in the last fiscal year, and that two-thirds of them agreed to return voluntarily to their home countries. But he was not able to account for the 4,400 people who did not choose to return home voluntarily.

Senator Carl Levin, a Michigan Democrat and chairman of the Senate's permanent subcommittee on investigations, responded, ''I find that disturbing, to put it mildly.''

Questioned by Senator Susan Collins, Republican of Maine, Mr. Pearson said it was true that the I.N.S. had no ability to verify that illegal immigrants who were let go but told to leave the country actually did leave.

Ms. Collins replied, ''If there's no system for checking if the individual has actually left in the 30 days as promised, isn't it likely they are not leaving?''

Mr. Pearson said, ''That could certainly be the case.''

A CLOSER LOOK
Expansion of Antiterrorism Efforts

President Bush's executive order regarding the trial of noncitizens for terrorist acts is only the latest action by the government to expand law enforcement activities in fighting terrorism. These are the other measures:

VISAS The State Department said last week it would slow down the process for granting visas to young men from Arab and Muslim nations.

LAWYERS The Justice Department said last week it would allow the authorities to monitor conversations between some people in federal custody and their lawyers.

DETENTION Under legislation passed by Congress last month, people held on the suspicion of being involved in terrorism can be held for up to seven days for questioning, after which they must be released if they are not charged with violations of the criminal or immigration codes.

WIRETAPS Law enforcement officials may now obtain from the special intelligence court the authority for roving wiretaps on a person suspected of involvement in terrorism so that any telephone used by that person may be monitored. Previously, officials needed separate authorizations for each phone used by the person.

WARRANTS Federal officials are now able to obtain nationwide search warrants for terrorism investigations.