Tens of thousands of persons from all over the country marched down Pennsylvania Avenue NW yesterday afternoon to rally at the Capitol and again decry the 1973 U.S. Supreme Court decision legalizing abortion as "the killing of babies."

"We are not going to brook this kind of massacre," Rep. Robert Bauman (R-MD) told the cheering crowd. Bauman comapred last year's one million legal abortions to the Nazi slaughter of Jews during World War II.

This was the fifth March for Life on the anniversary decision. This year, the marchers were younger - about of school age - and more openly Roman Catholic. Hundreds carried rosary boads and signs bearing religious messages.

There was at least one sign bearing the message "Lutherans for God, Country and Human Life Amendment," but there were many more reading "Pray the Rosary," and several huge banners proclaiming "Knights of Colombus - Rescue the Unborn."

Other signs carried messages such as "Abortion Deprives the Soul of Baptism," "Seminarians for Life," "Queen of Apostles March for Life;" "Pray the Rosary - Our Lady Asks You."

Twelve-year-old Chris Montello of Wevford, Md., said she took the day off from St. Margaret's parochial school where she is in the sixth grade to march because of "all the babies they're killing. We want to speak for the babies."

And Tom Rable, of Celina, Ohio, an eighth grade pupil at Immaculate late Conception School, was marching to "get a human life amendment" because abortion "is killing." He said his school "said I could come."

A 13-year-old pupil at St. Leo's School in Fairfax came to the march on one of two buses chartered by the school. Why did she come? "Beats me. Wait. Don't put that down!" she said. "I'm supposed to be marching to stop abortion because it's dumb."

The crowd estimates yesterday varied widely. U.S. Park Police counted buses from out of town, counted marchers passing by the White House, and put the number of protesters at 26,000.

The D.C. police counted buses, observed the crowd from a helicopter, timed how long it took the marchers to reach the Capitol from the Elipse behind the White House, and put the number at 70,000.

Reporters who covered this year's march and last year's, as well as numerous other demonstrations, felt the Park Police estimate was the more nearly accurate.

Marchers called repeatedly yesterday for the passage of a human life amendment to the U.S. Constitution, which would bar all abortions for any reason.

The past year was one of both great victory and setback for those opposed to abortion. After months of bitter, emotional debate and painful compromise. Congress passed a version of the so-called Hyde Amendment, which ends Medicaid funding for abortions except in the cases of pregnancy resulting from rape or incest, or cases where the mother's future health is threatened.

About 300,000 Medicaid abortions were performed last year, a figure likely to be vastly reduced by the funding cutoff.

But the antiabortion forces are outraged by the compromise. Nellie Gray, leader of the March for Life, said she expressed that outrage when she met for an hour and 15 minutes at the White House yesterday with President Jimmy Carter's domestic affairs' adviser, Stuart Eisenstadt.

"The life issue is not one for compromise and negotiation," Gray said she told the White House. "Either you're for killing babies or you're against killing babies."

The continued fight against abortion promises to be a volatile one again this year, as it has for the past 10 years. Another Hyde Amendment is anticiapted through which the antiabortion forces will try to totally eliminate federal funding for any abortions.

At the same time, the fight over public funding is moving into legislatures of some states that have continued to use state funds to pay for abortions for the poor.

Abortion is expected to be one of the major issues in both Maryland and Virginia during these sessions of those state's legislatures.

Shortly before the March for Life began yesterday, a group of persons favoring abortion held a press conference to announce the founding of a new organization for the restoration of federal funding for abortion.

Academy Award winning actress Estelle Parsons, one of those featured at the press conference, told reporters that she had obtained an abortion prior to the Supreme Court's ruling because she could afford to pay the necessary number of physicians to certify that an abortion was a medical necessity.

Parsons said she went "to a psychiatrist who was willing for money to send me to a doctor who was more than willing for money to send me to (other) doctors who were more than willing for money to send me to a private clinic, which was more than willing for money to anything I wanted.

"It's important for us to be here today to conteract that "March for Life," said Parsons, who was one of five speakers at the press conference.

The other four speakers were women who told of having had abortions themselves, or of friends who were denied abortions after the Medicaid funding cutoff.