President Reagan, speaking by loudspeaker hookup from the Oval Office, told a cheering crowd of more than 71,500 antiabortion demonstrators yesterday that "these days, as never before, the momentum is with us" to end "the terrible national tragedy of abortion."

At the same time, the president, addressing the annual March for Life rally on the 12th anniversary of the Supreme Court decision legalizing abortion, repeated his condemnation of the recent rash of bombings and arson at clinics across country.

"We cannot condone the threatening or taking of human life to protest the taking of life by way of abortion," Reagan said to applause from the shivering protesters, who gathered on the Ellipse before marching to the Supreme Court.

Twenty-nine demonstrators were arrested in a sit-in on the steps of the court -- the first planned civil disobedience there on the anniversary of its controversial decision.

Despite a temperature of 26 degrees with a wind chill factor of 1 degree above zero, the crowd was the largest ever at the annual rally and double last year's, according to U.S. Park Police estimates. The busloads of demonstrators carried signs proclaiming "Abortion: The Ultimate Child Abuse" and "Everyone Deserves a Birthday."

"I am proud to stand with you in the long march for the right to life," Reagan told the demonstrators, many of whom carried red roses, the trademark of the antiabortion movement, and wore black armbands commemorating "15 million babies aborted since 1973."

The march was the largest of hundreds of demonstrations taking place across the country on both sides of the emotional issue.

Among others addressing the crowd here were Moral Majority leader Rev. Jerry Falwell and a dozen congressmen opposed to abortion. As they listened to the speeches, some in the crowd debated what tactics should be used against what they viewed as the evil of abortion.

Minutes after Reagan spoke, two young marchers confronted an anti- abortion demonstrator who planned to be arrested at the "pray-in" on the steps of the Supreme Court, urging him to heed the president's warning against violence.

"I'm a right-to-lifer to the ultimate. But the president just asked us not to do anything illegal. Don't you bring the movement down," 19-year-old Chris Edwards, a political science major at New York's Niagara University, shouted at Tom Herlihy, 37, of Gaithersburg.

"We're just trying to to what Gandhi did, what Martin Luther King Jr. did," countered Herlihy, who was later arrested.

From the Ellipse, the marchers formed a human river more than four blocks long. Fifteen abreast, sometimes linking arms and singing, sometimes chanting "L-I-F-E, L-I-F-E," they marched down Constitution Avenue, past a Capitol still draped with inaugural bunting.

After the march, several thousand demonstrators visited offices of senators and congressmen to express their views. At the Supreme Court, 29 protesters, including a juvenile, were arrested when they knelt on the court steps and refused to leave, singing "All we are saying is give life a chance."

John P. Ryan, a St. Louis social worker who said he has been arrested 300 times for sitting in at abortion clinics, said he did not expect to be able to deliver a letter asking Justice Harry A. Blackmun, author of the abortion decision, to join their movement. Instead, Ryan said, his real hope was to get arrested again.

"He [Blackmun] doesn't have the courage [to meet with us]," said Ryan, who got his wish three hours later. "The justices have been able to hide from the consequences of their decision for 12 years."

Like some others in the small group who marched as pallbearers carrying a baby-sized coffin, Ryan said he would not condemn the bombings of 25 abortion clinic and related facilities across the country since last year's march.

"It's incredibly ironic that those guys are going to sit in jail for 12 or 15 years for destroying a building when for the last 12 years, 17 million babies have died in abortion chambers. Bricks and mortar -- 31 buildings -- it certainly pales in comparison to the babies," said Ryan, 32, who teaches workshops on how to sit in at abortion clinics.

Debbie Stockton, 25, of Arlington, helped carry the coffin from the Ellipse to the Supreme Court. But she said she was not going to try to get arrested this time because she is on probation for her arrest last Nov. 17 at the Metro Medical and Women's Center in Wheaton.

"I'm tired of being a martyr for the cause," Stockton said yesterday.

The first protesters arrested were Harry Hand, 27, and John Cavanaugh-O'Keefe, 34, both of Gaithersburg, leaders of the Pro-Life Non-Violent Action Project, which staged the pray-in at the court yesterday.

Those arrested were charged with violating a federal law that forbids picketing at the Supreme Court. The maximum penalty is 60 days in jail and a $100 fine.

Royce Lambreth, chief of the civil division in the U.S. attorney's office here, said yesterday that "these we would prosecute," unlike protesters at the South African embassy, who have generally had charges against them dismissed as lacking "prosecutive merit."

But U.S. Attorney Joseph E. DiGenova said later that he had not yet decided whether to prosecute the antiabortion protesters. Until prosecutors receive police reports about the arrests, "we cannot make judgments about the merits of these cases," diGenova said.

The centerpiece of the March for Life was Reagan's short address, the first time he has spoken directly to the marchers.

In previous years, he has met privately with antiabortion leaders, as he did again yesterday afternoon along with Health and Human Services Secretary Margaret Heckler, Surgeon General C. Everett Koop and other administration officials.

However, the White House, which places a premium on arranging "photo opportunities," refused to permit television cameras or photographers to record Reagan as he made the remarks.

At the daily news briefing, White House spokesman Larry Speakes refused to answer questions about that decision, saying, "That's just the way he wanted it done."

March organizers are pressing for passage of a Paramount Human Life Amendment to the Constitution that would bar all abortions, even when the life of the woman is in danger.

"We want the Paramount Human Life Amendment with no compromise, Mr. President," March for Life President Nellie Gray told Reagan.

"Good for you, and I support you," Reagan replied. In the past, Reagan has supported a constitutional amendment, but also said that he believes abortion should be allowed in situations where the mother's life is jeopardized.

The White House later issued a statement that Reagan did not mean to change his position on abortion.

Nanette Falkenberg, executive director of the National Abortion Rights Action League, criticized Reagan for focusing solely on "compassion for the 'unborn' " and trying "to make criminals of millions of women who will still obtain abortions" illegally if abortion is made illegal.

At a press conference, members of the Religious Coalition for Abortion Rights, which represents 31 national religious organizations, called for a "climate of peace and harmony" surrounding the abortion debate.

"Our Judeo-Christian heritage teaches us to resolve conflicts by nonviolent means, with love and respect for each other," said Fredrica F. Hodges, the group's executive director.