For the first time since they began arriving in this country more than 35 years ago from Adolf Hitler's death camps, thousands of Jewish Holocaust survivors will gather next week to recall their sufferings, search for lost friends and relatives and make a plea that one of humankind's darkest hours not be forgotten.

An estimated 10,000 survivors from the United States and Canada, along with their spouses and children, are expected here for four days of seminars, reunions and cultural events. These will include an address by President Reagan and a ceremony Wednesday morning on the west steps of the Capitol at which Vice President Bush will present the keys of two government buildings on the Mall that will house a permanent Holocaust Memorial and Museum.

In its size and level of official recognition, the gathering represents an unprecedented observance in this country of the events before and during World War II that led to the death of six million European Jews. The only comparable gathering took place in Jerusalem in 1981, and was attended by an estimated 8,000 survivors and their relatives.

"We have a tremendous story to tell the world not for our sake, but for the sake of the world, that it should never be repeated," said Benjamin Meed, president of the American Gathering of Jewish Holocaust Survivors, the sponsors of next week's events.

The most important aspect of the gathering, he said, will be "a collective message of the survivors to the world. A lesson to the world that it can happen again if we will not be watchful. If we will not pay attention to small incidents. Years ago they used to call it anti-Semitism; today they call it anti-Zionism, but actually it's the same thing."

The organizers of the gathering say they have several other purposes in holding the event: to thank the U.S. government for accepting so many Jewish survivors after the war and giving them an opportunity to reconstruct their lives; to provide living testimony that Jews resisted Nazi brutality as much as they could and that the Holocaust was real and not, as some revisionist historians have postulated, an exaggeration or the product of someone's imagination.

During and after the war an estimated 150,000 Jewish refugees came to this country, giving it the second largest survivor population in the world, after Israel. About 30 percent of those are believed to be dead.

"These people are very conscious of the passage of time and they want to speak about it before it's too late," said Lawrence Y. Goldberg, Washington director of the American Gathering of Jewish Holocaust Survivors.

"It's 40 years after the Holocaust and people have gone about their businesses and lived their lives and they have had their consciousness raised that time is going by and if we don't bear witness now, how will we do it tomorrow and who will pass it on to the next generation," said Rebecca Patt, another organization official.

The organizers also hope the gathering, which begins Sunday, will provoke a reevaluation of the image of those who survived the 2,351 Nazi concentration camps. For most people the term "Holocaust survivor" conjures up harrowing pictures of shaven, starving people in striped suits. But "that is a 40-year old image," said Goldberg. "What have they done since then? They have become rabbis, doctors."

Next week's events are primarily the brainchild of Meed, a Polish-born Jew who fought in the 1943 Warsaw Ghetto uprising and has been active in various Holocaust survivor organizations in this country. Two years ago, spurred by the gathering in Jerusalem, he began compiling a National Register of Jewish Holocaust Survivors, seeking names through a network of locally based survivor organizations. The register now has 40,000 entries, of which about 80 percent are survivors, according to Patt. The rest are their children.

Goldberg stressed that the gathering will be a strictly nonpolitical affair as far as domestic politics and Israeli-U.S. relations are concerned. "There is no legislative agenda, no head table, the message will come through on a personal basis," said Goldberg, who added that this was explained to the government of Israel, the country whose existence is a result of the Holocaust.

Israel's only formal involvement in the gathering is cultural. It has sent over an 85-member dance and orchestral ensemble called 'Anachnu Kahn' ('We are Here') and Dov Shilanskay, Israeli deputy minister without portfolio who is a survivor and who will speak at the opening assembly Monday night at the Capital Centre at which Reagan and Jewish Holocaust theologian Elie Wiesel are also scheduled to speak. Members of Congress also have been invited to attend the event.

Wiesel is chairman of the presidentially appointed U.S. Holocaust Memorial Council that is overseeing construction of the Holocaust museum. He will receive the keys to the two government buildings that will be its home from Bush. The council has endorsed the American gathering but is not organizing it, Goldberg said.

A ceremony is scheduled Wednesday at Arlington National Cemetery to thank the American military forces that liberated the Nazi concentration camps at the end of the war. There will also be continuous cultural and historical exhibitions in the Washington Convention Center and a program by those most directly affected by the survivors' experiences, their children.

The first-time reunion of so many people who share such traumatic memories is expected to make the gathering an extremely emotional affair. Many of those who come will be seeking someone who can provide details about the fate of their friends and loved ones lost without trace during the Holocaust.

"Still people are searching for each other," Meed said, explaining why they want to know about the last days of lost relatives. "Were they put to the gas chamber? . . .We have no cemeteries, we have no places, we really don't know. . . . You will see people looking in each others' eyes searching, searching for friends."

The names of all survivors, along with their spouses, children and grandchildren who register for the gathering, will be put into a master computer. Requests for specific persons or for information will also be programmed into a computer and then displayed continuously on television monitors.

By watching these television screens, organizers hope people will discover if someone is looking for them or is seeking information they have. They can then go to the computer center where they will be told how to contact the person who made the request.

In addition, signs with the names of over 200 cities, towns and villages in European countries that were under Nazi occupation will be posted on the pillars of the Convention Center so that survivors who share prewar roots can meet each other, Goldberg said.

The gathering will be paid for by the registration fees of those attending: $75 for each survivor and $50 for a spouse or child. "There is no corporation, no government, no foundation money," supporting the gathering, Goldberg said. About $125,000 was donated by the United Jewish Appeal Federation.

The local Jewish community has showed its support by offering volunteers to work at the gathering and more than 2,200 beds for those who are coming but cannot afford a hotel. The Capital Centre's fees were waived by owner Abe Pollin, while office space for the planners and the computers that will be used at the gathering were donated, Goldberg said.

"I have never seen a cause, except if Israel were threatened, a cause which could bring out the Jewish community like this one," Goldberg said, adding that some of the volunteers helping organize the event are Christians who walked in to offer unsolicited help.

The response of survivors exceeded the planners' expectations and even caused them some embarrassment. A few months ago the local Jewish community was told it would be more than welcome at all the events because the organizers were expecting only about 8,000 survivors. "But as we watched the response from the survivors mount, we realized we could not accommodate everyone," said Barbara Morgenstern, a volunteer from McLean. "We never expected 12,000 people."

Admission to the Monday night assembly will be by invitation only.

"These people are not bitter, though they have the right to be bitter . . . but this meeting is bittersweet," said Goldberg. "This is not a college or a camp reunion. To come is difficult. For many people it conjures up difficult memories. But they are not doing it defensively. Dignity is a very big word with them. They think of those who are not here."