"I know this is hard to believe," Nazi hunter Simon Wiesenthal says in the opening scene of "Genocide," "but there are those who will forget the Holocaust and there are some prepared to deny it."

Wiesenthal says this standing outside Mauthausen, the concentration camp where he was a prisoner some 40 years ago. "For me it was yesterday. I was a young man, just married, an architect by profession . . . "

The fear of not remembering, of course, is the reason to make and watch films like "Genocide." The film, which won the 1982 Academy Award for best documentary, opened here yesterday.

Given the subject matter this film isn't the most intense or the most extraordinary of its genre. That's both a shortcoming and a blessing. The film, which runs a mercifully short, quickly paced hour and a half, strives not just to show the death but also to impart some of the texture of European Jewish life before it was obliterated by the Nazis. The horrifying footage is there--the bulldozed bodies, faces eerily frozen in wide-eyed, doll-like stillness, the women, standing on the edge of open graves, forced to strip before being shot into the graves. But the film doesn't hit you over the head with it.

Although the film is not as brutally graphic as it could have been, it makes no bones about being harsh on the U.S. and its allies. Basically, they did nothing, the film contends. "In the whole history of false dawns," thunders narrator Orson Welles, "the news that the U.S. was involved in the war against Germany was perhaps the falsest dawn." The film shows ships of Jewish refugees being turned away at U.S. and Cuban ports. It also shows how newspapers of the day underplayed the stories of the exterminations.

The film is narrated quite effectively by Orson Welles and Elizabeth Taylor, both of whom donated their services. Taylor is a member of the board of the Simon Wiesenthal Center, which was a coproducer of the film. Frank Sinatra donated $100,000 to make the $2.5 million film, which has been shown only once before in Washington, last year at the Kennedy Center. There was a benefit screening Thursday evening at the Janus Theater, sponsored by the American Jewish Congress and the Wiesenthal Center. For the rest of its run, proceeds from the film will go to the Wiesenthal Center and the distributor, United Artists.

The quick historical overview ("In every society, Jews were the convenient scapegoat.") of "Genocide" is rather simplistic, but there are interesting touches--such as the picture of a child's board game called 'Juden Raus!' (Jews Out!) as an example of how virulent and ubiquitous anti-Jewish propaganda had become in Germany of the 1930s.

All of the film is either newsreel footage or graphics--the latter are elaborate but oversimplified and reminiscent of educational film strips shown in school. (During the early part of the film, you half expect to hear the beep that tells the teacher it's time to roll the next frame.)

What does emerge from the film, distinguishing it from others, is a picture of a defiant, not sheeplike, Jewish population that went to the concentration camps. There is the story of a Jewish woman working in a munitions plant in Auschwitz who managed to smuggle out dynamite that was used to blow up a crematorium. Six hundred Jews escaped. Most were found by their German captors and the woman was hanged in the camp. Her last words, scribbled on a piece of paper, were, "Be strong and be brave." .

Most powerful--and saddest--in "Genocide" is the testimony of survivors, culled from archives and narrated by Taylor (you never see her; you just hear her voice).

In one segment, survivor Leon Kahn relates how he fled Poland, leaving his mother behind.

"I was torn between my terror and my conviction that I was betraying her," wrote Kahn, whose words are read by Taylor. "One hundred times I told myself to go. One hundred times I told myself I should stay. To this day, I wonder if we had tried to convince her more, she would have come . . . " His mother died in a concentration camp.

The testimony is accompanied by dramatized noises (usually screams and cries of anguish) and often real footage from the time, once of people actually being shot in the distance. Though intense, it's not unwatchable and the film pulls away to lower-key segments where the viewer can catch his breath.

Near the end, a stunned BBC newscaster reports on the wrenching sight of the Allied forces liberating Bergen-Belsen. His words could well sum up the theme of the film:

"This is what the Germans did," he says. "Make no mistake about it--did deliberately and slowly to doctors, authors, lawyers, musicians and professional people of every kind . . . This day at Belsen was the most horrible of my life."