President Reagan on Wednesday joined in laying the cornerstone for the U.S. Holocaust Museum, calling it a "saddening but triumphant occasion" and vowed that the victims of the Nazi annihilation of Jews will not be forgotten.

The museum, dedicated to the more than 6 million Jews and other victims killed by the Nazis in World War II, is expected to cost $147 million for construction and furnishing. Completion of the building, to be located not far from the Washington Monument, is planned for late 1990.Reagan reminded the audience that "tens of thousands of Jews" want to leave the Soviet Union. In a challenge to the Soviet leadership, the president asked: "Where are those exit visas? Where are they?"

"I say let these people go," Reagan said.

"We gather here today here for a solemn, saddening, yet triumphant occasion, an occasion that commemorates the seriousness of our intention that human beings, as Americans and as many here today as Jews, to keep the memory fresh and enduring" of the victims, he said.

"We who did not go their way, owe them this."

Reagan dramatically recalled the horrors of the concentration camps in Europe during World War II, calling them the "image of the inferno."

"I believe the Holocaust is comprehensible," he said. "The museum will make it impossible to forget."

Reagan said the museum will study the history of the Holocaust, "bringing together in one place" the story of the Jewish genocide "and the curse of anti-Semitism."

He denounced those who deny "the truth of the Holocaust" and said that the challenge "must be held up to scorn" at home and abroad.

Reagan also denounced the "infamous" U.N. resolution "equating Zionism with racism" and called for its repeal.

The museum's permanent exhibit will tell the history of the Holocaust, from its beginnings in 1933 through its aftermath, the achievement of its survivors and its impact on philosophy and theology.

The museum will be situated off Raoul Wallenberg Place, a street near the Bureau of Engraving renamed in honor of the Swedish businessman and diplomat who became a legend through his efforts to rescue thousands of Jewish refugees during World War II. Wallenberg disappeared while a prisoner in the Soviet Union.

The Soviets have said Wallenberg died of a heart attack in a prison cell in 1947, but there have been numerous unconfirmed reports by freed Soviet prisoners that he had been seen alive in prison.

Congress, on Sept. 22, 1981, granted honorary citizenship to Wallenberg.

Reagan is serving as honorary chairman of "A Campaign to Remember," which collects funds for the museum. To date, $50 million has been donated for the memorial.

In 1980, Congress established the U.S. Holocaust Memorial Council, whose 55 members are charged with sponsoring and encouraging annual Days of Remembrance events nationwide for the victims of the Holocaust.