Chanting and singing and buoyed by President Reagan's personal endorsement, more than 50,000 abortion opponents marched down Pennsylvania Avenue to the U.S. Supreme Court Building yesterday to protest the court's decision 15 years ago legalizing abortions.

The president, declaring that all human life is sacred, announced in a telephoned speech to the group that his administration soon will publish its previously promised regulations barring family planning clinics from using federal funds to offer abortion counseling or make referrals to abortion services.

Too often, said Reagan, whose remarks were heard over a loudspeaker at the Ellipse, the clinics tell women that abortion "is their best, if not their only option."

With the new regulations, Reagan said, there will be "no mingling of silver," and family planning groups will have to be physically and financially separate from those groups involved in "abortion-related activities."

Family planning organizations have threatened to sue to block the regulations, which a spokeswoman for the Planned Parenthood Federation of America yesterday called "a violation of free speech."

Nellie Gray, an antiabortion leader and "March for Life" organizer, took the opportunity to scold Reagan, while she had him on the line, for signing a budget bill that includes abortion funds for the District.

"Well, Nellie," the president responded, "sometimes these things happen because, as you know, there are people that are in great disagreement with us. But we are continuing to work and to do our best to end any federal funding."

The demonstrators, carrying signs, banners and photographs of aborted fetuses, held a rally on the muddy Ellipse, then marched past the White House to Capitol Hill, where police said 35 persons were arrested during a civil disobedience protest on the steps of the Supreme Court Building.

The signs and banners -- "Stop Killing Babies," "Orthodox Christians for Life," "Stop the American Holocaust" -- were carried by teen-agers and parents, including adoptive parents, and by priests, nuns and other representatives of religious denominations.

Many of the demonstrators wore or carried red roses, which have become the symbol of the antiabortion movement.

But Monica Moore, who described herself as a Christian homemaker from Wooster, Ohio, carried an inflated plastic dinosaur and the sign, "Make Abortion Extinct." The 47-year-old mother of six said she had traveled by bus to attend the Washington march for the second time.

"I just really believe that as a Christian, it's my responsibility to speak against abortion," said Moore, who said her active involvement in the antiabortion movement began in 1983 after reading a book that described the "heart-breaking" procedure.

Earlier yesterday, the Religious Coalition for Abortion Rights held a "Speak-Out" on the steps of the Supreme Court in support of the 1973 Roe v. Wade decision, which found that laws banning abortion violated a woman's right to privacy.

"We want to protest absolutism in thought, extremism in action, violence, harassment and terrorism," said Frederica Hodges, executive director of the coalition, a national organization of 31 Protestant and Jewish groups.

"As we end the bicentennial year celebration of the U.S. Constitution, it is imperative that Americans recognize the importance of working to protect our most basic freedoms," Hodges said. "These must, of necessity, include religious and reproductive freedoms."

At the antiabortion rally, Rep. Robert Dornan (R-Calif.) attacked recent experiments that he said had used human livers from aborted fetuses to treat brain and nerve disorders.

"This battle has now reached the Frankenstein stage," Dornan shouted, as the crowd applauded in agreement.

Rep. Michael DeWine (R-Ohio) said he hoped the Roe v. Wade decision would soon meet the same fate as the court's 1857 Dred Scott decision, which affirmed the practice of slavery.

"That was a horrible mistake, but it was overturned," DeWine said.

Marching past the Justice Department, demonstrators roared their approval when a man leaned out of a fourth-story window of the building and gave them a thumb's up sign, inspiring chants of "Life Yes, Abortion No."

After a meeting with Reagan, Kathy DiFiore, founder of a New Jersey group that cares for unwed mothers, said she was untroubled that he has been unable to get abortions outlawed.

"Ronald Reagan is our hero," she said. "He did everything he humanly could to overturn this terrible law . . . . I only hope he is followed by someone who has the courage to do the same thing."