ASHINGTON, Sept. 18 ・The Bush administration today announced a major expansion of its power to detain immigrants suspected of crimes, including new rules prompted by last week's terrorist attacks that would allow legal immigrants to be detained indefinitely during a national emergency.

Citing the new powers, the Justice Department said it would continue to hold 75 immigrants arrested in connection with the attacks on the World Trade Center and the Pentagon. Previously, the department faced a 24- hour deadline on whether to release detained immigrants or charge them with a crime, or with violating the terms of their visa.

The new detention powers drew statements of concern from civil liberties advocates and immigration lawyers. While the pressure on the administration and Congress to act is immense in the wake of the terrorist attacks, there is rising concern on the left and the right that the rush to respond could erode basic constitutional freedoms.

The administration, which had the authority to rewrite the detention rules, is also expected within days to present Congress with a broad package of anti-terrorism legislation. Civil liberties and privacy groups are pleading with Congress not to act hastily on the package.

A draft bill circulating today on Capitol Hill, apparently reflecting the administration's views, would give new authority to the Justice Department to arrest immigrants suspected of terrorism, accelerate the process of deporting them and curtail court appeals.

In announcing the new regulations, Attorney General John Ashcroft said at a news conference that the government had "a responsibility to use every legal means at our disposal to prevent further terrorist activity by taking people into custody who have violated the law and who may pose a threat to America."

Mr. Ashcroft insisted that "we're going to do everything we can to harmonize the constitutional rights of individuals with every legal capacity we can muster to also protect the safety and security of individuals."

Under its new powers, the Immigration and Naturalization Service, which is part of the Justice Department, would normally have 48 hours to decide whether to release or charge a detained immigrant.

The 48-hour deadline could be waived, however, "in the event of emergency or other extraordinary circumstance," allowing an immigrant to be held for "an additional reasonable period of time" without charges.

The new rules would apply to immigrants and foreign visitors who entered the country legally but who are suspected of committing crimes in the United States, or who have overstayed a visa or violated other terms of their entry into the country. They would not apply to citizens.

David Martin, a law professor at the University of Virginia and a former general counsel of the Immigration and Naturalization Service, said "there's definitely a civil liberties concern" in the new regulations.

"I don't want to be alarmist about this," Mr. Martin said. "If we're talking about adding an additional 12 hours or 24 hours to detention, I don't think that's a problem. But if we are holding people for weeks and weeks, then I think there will be close scrutiny."

Jeanne A. Butterfield of the American Immigration Lawyers Association said that in the midst of the crisis created by last week's terrorism, the new rules "may be reasonable, but no one wants to see this lead to some kind of indefinite detention."

The Supreme Court has questioned the constitutionality of indefinite detention, ruling last summer that the government could not order open- ended detention of illegal, clearly deportable immigrants simply for lack of a country willing to take them.

Human rights groups have long criticized indefinite-detention laws in other countries, noting that they are often used by repressive governments to lock up dissidents for months or years under the guise of "emergency" conditions.

The Justice Department has announced that it will ask Congress for broad new surveillance authority to place wiretaps on phones and computers and a variety of other powers to fight terrorism. Mr. Ashcroft has asked Congress to act within days, and few politicians or advocacy groups have been willing, until now, to suggest a more cautious response.

But in recent days, more are stepping forward to urge lawmakers and the administration to slow down, examine the security flaws that led to the attacks and consider the consequences of various proposals for civil liberties.

Representative Bob Barr, a Georgia Republican and staunch opponent of gun control, sent a letter to Mr. Ashcroft and Congressional leaders today declaring, "Before we begin dismantling constitutionally protected safeguards and diminishing fundamental rights to privacy, we should first examine why last week's attacks occurred."

Senator Patrick J. Leahy, the Vermont Democrat who is chairman of the Senate Judiciary Committee, acknowledged in an interview that there was "a hunger to act," but added, "My concern is that at some point you've got to stop doing things that give you a nice press release and start doing things that actually protect the nation."

Mr. Leahy promised quick ・but careful ・action, adding, "The first thing we have to realize is this is not either or ・this is not the Constitution versus capturing the terrorists. We can have both."

And Morton H. Halperin, a longtime official with the American Civil Liberties Union and a veteran of the State Department under the Clinton administration, described Mr. Ashcroft's plea for action by week's end as "deeply troubling."

Mr. Halperin, now a senior fellow with the Council on Foreign Relations, added, "We should not be enacting restrictions on the liberty of Americans without careful debate. If we do it carefully, we can find an acceptable balance. If we rush into it, we will do things that deprive people of their liberty without improving security."

Mr. Halperin was one of the organizers of a coalition of groups, as varied as the American Civil Liberties Union and Americans for Tax Reform, that met for the first time on Friday. That coalition, informally known as the In Defense of Freedom Coalition, has scheduled a news conference for Thursday to declare its concerns. At the moment, many of these groups are demanding that the administration's proposals be the subject of open debate and orderly consideration.

Grover Norquist, the influential conservative strategist who heads Americans for Tax Reform, said, "I've heard some politicians say we need to pass this this week ・that's code for, if anybody read it, it wouldn't pass."

Jerry Berman, executive director of the Center for Democracy and Technology, another member of the new coalition, said, "We've had a history of racing to judgment and passing inappropriate and wrongheaded and ultimately counterproductive laws."

Mr. Berman added, "Before you pass legislation in this area, you need to know what happened. And I have not yet found a story or a statement by any official that says the failure here was caused by restrictions on electronic surveillance."

But Ron Klain, who served in the White House and Justice Department under President Bill Clinton, argued that "it's inevitable at times like this that the pent-up agenda of law enforcement gets put forward."

"I think there is nothing wrong and probably something right about Congress acting on these matters relatively quickly," Mr. Klain said.

Moreover, he said, the public may well be ready to recalibrate the balance between civil liberties and security. "The Constitution is not a suicide pact," he said.

Mr. Klain argued that Congress needed, however, to ensure that whatever trade-off was made actually resulted in a safer society.