ASHINGTON, Oct. 25 — Lawmakers today hailed the antiterrorism legislation passed by Congress as an appropriate balance between civil liberties and the heightened security needs of a nation at risk. But civil liberties advocates asserted that the new law jeopardized fundamental rights and would require vigilant oversight to keep the authorities in line.

Law enforcement authorities said they would move quickly to capitalize on new powers for the investigation, surveillance and detention of suspected terrorists.

"Today, the Department of Justice is positioned to launch a new offensive against terrorism," said Attorney General John Ashcroft, promising "airtight surveillance of terrorists."

The reactions only underscored the reality of a lopsided vote in the House and the Senate: In this latest struggle between civil liberties advocates and law enforcement, the authorities had won. The pattern was a familiar one, many legal experts say: The federal government had reached for new powers in a time of crisis, and the Congress had complied, as it did after the Oklahoma City bombing in 1995.

But civil liberties advocates say there is another historical parallel: that new powers granted in a crisis are ripe for abuse. The new standards for surveillance and investigation are so flexible, so weighted toward the government, these advocates say, that the authorities are almost certain to use such powers against law-abiding people.

The Federal Bureau of Investigation "is under great pressure," said Jerry Berman, executive director of the Center for Democracy and Technology, an advocacy group. "It will be under pressure from the press, from policy makers, to leave no stone unturned. And unable to pierce the inside of a sophisticated terrorist cell, I'm afraid they're going to spend a lot of time on the outside of the circle, which includes many people who are not engaged in anything unlawful."

Privacy advocates like Mr. Berman note that the bill gives the government the ability to obtain an array of records and information about individuals, with little judicial oversight. It also allows law enforcement authorities to share information obtained in grand jury investigations and elsewhere with intelligence authorities.

And civil libertarians say they have great concern about the expansion of the powers of a special court that grants secret authorization for the government to conduct electronic surveillance against people suspected of involvement with a foreign government.

Laura W. Murphy, director of the American Civil Liberties Union national office, also argued that the new law defined the crime of harboring and supporting terrorists so broadly that it was bound to have a chilling effect on political activity by Americans in antiwar or pro-immigrant groups.

This debate was on vivid display at a legal conference today in a joint appearance of several civil liberties lawyers and Viet Dinh, an assistant attorney general in the Bush Justice Department who helped draft the bill.

David Cole, a lawyer with the Center for Constitutional Rights who has represented Arab-Americans in national security cases, argued that the law was heavily weighted against immigrants. It was also written so broadly, Mr. Cole said, that it would make people eligible for deportation for supporting groups that are only marginally involved with terrorism but are primarily charitable or political organizations. For example, he asserted, the African National Congress was at times labeled a terrorist organization by the United States government, but was nevertheless very popular with many Americans.

Eliot Mincberg, the legal director of People for the American Way, a liberal group, asserted that the legislation was essentially saying, "We're the government. Trust us."

But Mr. Dinh of the Justice Department countered, "We're not saying you should trust us. I haven't used those words. I do not think these powers will be abused nor can they be abused." Mr. Dinh said that, in the end, the Constitution's protections will still apply — and that the courts will have the final say on many parts of the legislation.

Those who support the legislation — including many lawmakers traditionally identified as supporters of civil liberties — note that some of the most controversial provisions for surveillance authority will expire in four years under a sunset provision, unless Congress chooses to renew them.

Representative Howard L. Berman, a California Democrat with an 85 percent rating from the A.C.L.U., was among those voting for the bill. "It would not have passed pre-Sept. 11," he said. But Mr. Berman was one of many lawmakers who argued, obviously, that the world changed on Sept. 11. "I always have concern that what I think are important new powers will be misused," he said. "But to me the most important safety valve we have here is the sunset. We have the sunset, we have the media, and presumably we'll have Congressional oversight."

But Elisa Massimino, the director of the Washington office of the Lawyers Committee for Human Rights, said, "These kinds of provisions, once they infect a country's justice system, are incredibly hard to cure."

She noted that many of the emergency powers that were put into place in Northern Ireland temporarily have never been lifted, and have only been expanded to the rest of the United Kingdom.

Still, Mr. Berman's attitude prevailed. He argued, in an interview a few weeks ago, that this was not about the government monitoring or inhibiting political activity, which many critics asserted was the real agenda of much of the surveillance of the 1960's.

"This is more like war," he said.