President Bush signed into law yesterday the most significant federal restriction on abortion in the 30 years since the Supreme Court's Roe v. Wade decision established constitutional protection for terminating pregnancies.

The new prohibition is for a relatively uncommon procedure -- what opponents call "partial-birth abortion" -- but both sides in the debate viewed it as a symbolic shift by the federal government against abortion after an eight-year legislative struggle and two vetoes by President Bill Clinton.

"America stands for liberty, for the pursuit of happiness and for the unalienable right of life," Bush said to a cheering crowd in the Ronald Reagan Building before a signing ceremony that honored the men who shepherded the legislation through Congress. "This right to life cannot be granted or denied by government because it does not come from government, it comes from the creator of life."

In an indication of the potential sweep of the new law, Attorney General John D. Ashcroft assigned responsibility for enforcing it to the Justice Department's civil rights division rather than its criminal division. Officials said the decision, revealed to prosecutors yesterday, broadens the civil rights protection of fetuses but creates a potential conflict of interest for the civil rights division's criminal section. That section is also responsible for prosecuting those who block access to abortion clinics.

The new law was immediately challenged in court. A federal judge in Nebraska issued a narrow restraining order against the law in a case brought by four doctors, while abortion rights supporters challenged the ban in New York and California. Bush, anticipating legal challenges, won an extended ovation from abortion foes when he vowed to fight the lawsuits.

"The facts about partial-birth abortion are troubling and tragic, and no lawyer's brief can make them seem otherwise," he said, adding: "The executive branch will vigorously defend this law against any who would try to overturn it in the courts."

The law bars doctors from an "overt act," such as puncturing the skull, to kill a partially delivered fetus. The ban has an exception to protect a woman's life, but not her health, which Congress determined was not necessary. The Supreme Court three years ago struck down a similar law in Nebraska, but architects of the federal ban said they have taken into account the court's objections.

Bush has said the nation is not ready for a more comprehensive abortion ban, but abortion opponents were optimistic. "President Bush's signing of the Partial-Birth Abortion Ban Act marks the first time in three decades that our nation has placed any restriction on an abortion procedure," said Archbishop Charles J. Chaput, chairman of the U.S. Conference of Catholic Bishops' Committee for Pro-Life Activities. "This is a vital step in the right direction."

Abortion rights advocates were equally sweeping in their denunciations. "Today George W. Bush sends a message to every woman and girl in the United States: Your reproductive rights are not guaranteed," said Kim Gandy, president of the National Organization for Women (NOW). "By signing the deceptively named Partial-Birth Abortion Ban into law, Bush confirms that his administration and Congress have both the power and the will to overturn Roe v. Wade, one step at a time."

A planned protest by NOW produced a modest turnout. A few dozen demonstrators could be seen holding signs with messages such as "Keep Abortion Legal" as Bush's motorcade traveled to the Reagan Building for the brief ceremony.

Opponents of the ban have turned their efforts from the legislative branch to the judiciary. Less than an hour after the signing, U.S. District Judge Richard Kopf in Lincoln, Neb., issued a temporary injunction that applied only to the doctors who brought a lawsuit challenging the law. Kopf took issue with the lack of an exception in the law to protect a woman's health -- the grounds Clinton cited in vetoing similar legislation.

"While it is also true that Congress found that a health exception is not needed, it is, at the very least, problematic whether I should defer to such a conclusion when the Supreme Court has found otherwise," the judge said.

In the Reagan Building, there was only extended cheering for Bush, who smiled and nodded during frequent interruptions for applause. "For years, a terrible form of violence has been directed against children who are inches from birth while the law looked the other way," the president said, standing before a bank of flags. "Today, at last, the American people and our government have confronted the violence and come to the defense of the innocent child."

Abortion foe John Sargent confronts a group protesting the ban. A few dozen people took part in the National Organization for Women's demonstration.