The Supreme Court, rejecting pleas of the Reagan administration, yesterday strongly reaffirmed its 1973 decision legalizing abortion and struck down by a 5-to-4 vote a Pennsylvania law that the justices said attempted to "intimidate" women seeking abortions.

Justice Harry A. Blackmun, who wrote the 1973 decision, Roe v. Wade, announced the opinion from the bench yesterday, raising his voice as he said, "We reaffirm once again the general principles" of that decision.

The court invalidated a Pennsylvania law that required doctors, before providing abortions, to tell women of the "detrimental physical and psychological effects" of abortion, to describe various stages of fetal development, and to discuss aid that might be available if she decided not to have an abortion.

The law also required physicians to provide the state with extensive reports on each abortion, to have two doctors present during late-pregnancy abortions and to use methods least risky to the fetus' possible survival.

Blackmun said a "close analysis of those provisions" shows they were designed "to deter a woman from making a decision that, with her physician, is hers to make."

"The states are not free, under the guise of protecting maternal health or potential life, to intimidate women into continuing pregnancies," he said, upholding a Philadelphia appeals court ruling.

Blackmun said the Constitution "embodies a promise that a certain private sphere of individual liberty will be kept largely beyond the reach of government . . . . Few decisions are more personal and intimate, more properly private, or more basic to individual dignity and autonomy, than a woman's decision . . . whether to end her pregnancy."

Dissenting were Justices Sandra Day O'Connor, Byron R. White, William H. Rehnquist and Chief Justice Warren E. Burger, who for the first time said he thought it was time to "reexamine" the 1973 decision.

The administration, in a controversial brief, had asked the court not only to allow the Pennsylvania restrictions but also to overrule Roe v. Wade, even though it was not explicitly at issue in the case.

Solicitor General Charles Fried, in an unusual news conference after the decision, called the ruling a "defeat," but said the brief sparked a "serious and important constitutional discussion" of the issue.

Fried said that, barring a signal from the justices that they were prepared to change their minds, he felt it would be "slightly futile" to try again. "There is such a thing as making a pest of yourself and repeating yourself," Fried said. "I have no inclination to be a nag."

The ruling yesterday, coming on the heels of the court's decision Monday rejecting the Reagan administration's "Baby Doe" rules for the treatment of handicapped infants, is the second major defeat for the administration this week in its drive to use the courts to further its social agenda.

Fried and antiabortion groups said they were encouraged by what they saw as a shift in position by Burger, who voted with Blackmun in 1973 but dissented yesterday. Burger, long considered a wavering member of the original seven-member majority in Roe, had dissented in the past as the court struck down various state regulations.

But yesterday Burger said that "we should reexamine Roe" and said the court had gone too far in limiting the power of states to regulate abortions.

White, who dissented in Roe, said the 1973 decision was "fundamentally misguided since its inception," called for it to be overruled and for states to be allowed once again to regulate or ban abortions.

"Abortion is a hotly contested moral and political issue. Such issues, in our society, are to be resolved by the will of the people, either as expressed through legislation or through the general principles they have already incorporated into the Constitution they have adopted," White said. "I would return the issue to the people by overruling Roe v. Wade."

White, in a 29-page dissent, said yesterday's ruling "appears symptomatic of the court's own insecurity over its handiwork" in Roe v. Wade, arguing that the Pennsylvania provisions "in no way contravene the right recognized in Roe. I do not share the warped point of view of the majority . . . . "

O'Connor, Reagan's only appointee on the court, also dissented from the court's 1983 ruling invalidating restrictions on abortions in a case from Akron, Ohio. O'Connor said yesterday the issue before the court was not really abortion, but a procedural controversy over whether lower federal courts acted properly in striking down the disputed parts of the law.

She said the appeals court overstepped its authority in voiding the provisions and accused the majority of bending its own rules of legal procedure in order to affirm the appeals court. "What is at issue here," she said, "is a matter of legal principle . . . . If this case did not involve state regulation of abortion, it may be doubted that the court would entertain, let alone adopt, such a departure from its precedents."

O'Connor said the court "appears to adopt" a new test under which "any regulation touching on abortion must be invalidated if it poses 'an unacceptable danger of deterring the exercise of that right.' "

Observers on both sides said yesterday's 5-to-4 ruling, despite the comments of Burger and O'Connor, did not mean that a one-vote shift could overrule Roe v. Wade.

Harvard Law School Professor Laurence H. Tribe, who filed a brief urging the court to reaffirm Roe, said neither O'Connor's nor Burger's opinions indicate they would be willing to join White and Rehnquist and allow states to ban abortions.

It would be a "wholly unjustified leap," Tribe said, "to say that Justice O'Connor would recognize no right to abortion."

Edward R. Grant, executive director of the antiabortion Americans United for Life, said Burger's comment about reexamining Roe was a "significant development."

But Grant said "nobody who works in this area expects Roe v. Wade to be overruled in an instant. You lay brick on brick." A shift of one or two votes could "make a lot of progress" in limiting Roe, but not necessarily in overturning it, Grant said.

Abortion rights advocates immediately hailed the ruling. Lynn Paltrow, an attorney with the American Civil Liberties Union, called it a "tremendous pro-choice victory," and Philadelphia attorney Kathryn Kolbert, who argued against the law before the Supreme Court, said the court had "protected individual rights of women against an increasingly vocal and hostile antiabortion minority."

Abortion opponents, such as Dr. John Willke, president of the National Right to Life Committee, said, "It will take new people on the court to make a difference." Pennsylvania state Rep. Stephen Freind, a sponsor of the law at issue yesterday, said he would review the decision and try to pass new legislation restricting abortion. "There are no such things as defeat when you are pro-life," he said. "There are only temporary setbacks."