Forty years ago, American troops who liberated the Nazi death camps found scenes of horror: mass graves, gas chambers and skeletal survivors. Yesterday in the splendor of the Capitol Rotunda, where 10 flags of the liberating U.S. Army divisions were held high, the mood was one of remembrance.

Survivors of the camps, members of Congress and others listened as Secretary of State George P. Shultz declared, "I share with you also the deep conviction that there is no place within the deep spirit we feel of reconciliation . . . for understanding for those who took part in the perpetration of the Nazi horror."

Shultz's statement at the official U.S. ceremony for the "Days of Remembrance," the April 14-21 commemoration of the World War II slaughter of 6 million Jews and millions of other noncombatants, was greeted with strong applause. The secretary's remarks coincided with intense pressure on President Reagan to abandon his plan to lay a wreath at a German military cemetery at Bitburg, where 30 members of the infamous Waffen SS are among those buried.

Yesterday at the solemn Rotunda ceremony, author and scholar Elie Wiesel, a survivor of the Auschwitz and Buchenwald death camps and now chairman of the U.S. Holocaust Memorial Council, said:

"Auschwitz was conceived, structured, elaborated, perfected, built, organized and implemented by the SS. They were the killers of Jews primarily, but not only of Jews. They butchered Poles and Czechs, French and Dutch, Norwegians and Danes, Yugoslavs, Ukrainians, Greeks, Gypsies and gays. Auschwitz was a universe and the SS were its gods. Why, then, should anyone visit and, by doing so, honor their cemetery as though they had been nothing but patriotic soldiers who died for their fatherland?"

The guests applauded loudly when the battle flags of American Army divisions were paraded into the hall by members of the 3rd U.S. Infantry, "The Old Guard." As the flags came in one by one, a young trooper called out the name of the unit and the camp liberated -- "6th Armored Division, Buchenwald . . . 42nd Infantry Division, Dachau . . ."

The 10 flags were presented to the council and are to be displayed in the planned U.S. Holocaust Memorial Museum.

Said Council Vice Chairman Mark Talisman, "Our American troops, joined by the Allied forces, opened the very gates of Hell 40 years ago." Army Secretary John O. Marsh Jr. said, "The flags that you see here speak louder than any words I might say."

Wiesel noted that the SS "slaughtered defenseless American war prisoners at Malmedy." With Shultz sitting a few feet away waiting for his turn to speak, Wiesel asked him to intercede with the president: "I plead to you, please be our emissary. Tell those who need to know that our pain is genuine, our outrage is deep and our perplexity is infinite."

Both Wiesel and Shultz received standing ovations from the several hundred guests at the Rotunda ceremony, including most of the 65 members of the Holocaust Council, chartered by unanimous votes of both houses of Congress to conduct the annual Days of Remembrance and build an official U.S. Holocaust Memorial Museum with $100 million in private funds on government land near the Washington Monument.

In the United States, there are official commemorative ceremonies planned in all 50 states. Thousands gathered for a ceremony in Madison Square Garden in New York City Wednesday night, and in Philadelphia's Civic Center next Sunday there will be a "commemoration through the performing arts" with James Earl Jones, F. Murray Abraham, Martina Arroyo, the Singing City Choir and others as part of a three-day American Gathering of Jewish Holocaust Survivors.

In Israel yesterday, where this year the Knesset had designated April 18 as the annual Yom Hashoah -- "Day of the Holocaust" -- traffic stopped and citizens stood at attention, heads bowed, as air raid sirens signaled two minutes of silence.

In the Rotunda yesterday, Sen. Claiborne Pell (D-R.I.) said monstrosities like those of the Nazi camps must be stopped "by all of us not standing by when we see the start of evil."

Six candles were lit for the 6 million dead. A man in the audience, representing Gypsy Holocaust victims, lit a seventh candle and held it aloft for a time. Gypsy spokesman Danny Bimbolet of Boston said in an interview yesterday that a Gypsy representative should be included on the Holocaust Council. Talisman said the members are presidentially appointed and that efforts are being made to include a Gypsy member.

Wiesel attacked the Soviet Union for anti-Semitism, saying that "the word 'Jew' hardly appears" at death camp sites in the Soviet bloc countries. "Shame on Moscow," he said, "shame on the Communist rulers of East Germany, for they twist and falsify history."

Shultz in his speech said that Nazism "dealt an almost devastating blow to all our most fundamental hopes for the modern world. Those who prior to the war had maintained their faith in the possibility of human progress, in the idea that with high culture and high civilization would come the end of man's inhumanity to man . . . were stunned by the Holocaust."

Guests at the Rotunda yesterday also included Sen. Paula Hawkins (R-Fla.); Reps. Bill Green (R-N.Y.), Stephen J. Solarz (D-N.Y.) and Sam Gejdenson (D-Conn.), the son of a Holocaust survivor; Interior Secretary Donald P. Hodel; and a large number of scholars, theologians and other dignitaries who are members of the Holocaust Council.

Controversy over Reagan's trip has touched the many events of the Days of Remembrance this year. Concentration camp survivors and others condemned the Bitburg visit at a museum fund-raising reception here Wednesday night. At a remembrance breakfast yesterday in the Dirksen Senate Office Building, Sen. Frank R. Lautenberg (D-N.J.) gave a speech linking Reagan's proposed cemetery visit to forgetting and even "denial" of the Holocaust.

"The president inadvertently would contribute to that denial if he fails to visit a concentration camp site -- not a synagogue -- but a concentration camp site, and eliminate the stop at the cemetery," said Lautenberg. " . . . Whatever bizarre good could come of it is offset by the fact that he pays tribute to murderers."

Before yesterday's ceremony, Wiesel said of Reagan, "It is not a contest. We are not on different sides . . . but there is no reconciliation with the SS. There can never be."