It was a party for a group of American Jews who had been on vacation together. But the Tuesday evening slide show at the Potomac home of Washington Hebrew Congregation's Rabbi Joseph Weinberg and his wife Marcia was not the usual array of landscapes of Europe, the Caribbean, Florida or other leisure spots.

The pictures were of Soviet Jews in and near Moscow, an increasingly popular destination for American Jews who are being encouraged to make the trips by a U.S. network of Soviet Jewry sympathizers.

The main reason for these trips, as it was with the Weinbergs, is to make contact with what has become known any of the (tourist) groups were picked of Soviet Jews who have been refused permission to migrate to Israel.

Rather than lying on the beach or tramping through museums, these tourists pass the time by avoiding Soviet officials (referred to as "KGB men") whose notice they have invited by passing goods and news to the "refusedniks," who often are under surveillance by the Soviets.

This time, three in the group said they were picked up and questioned by Soviet officials while attending a picnic with 250 "refusedniks" in a birch forest 30 miles outside Moscow. After being questioned aboard the bus outside a "police building" they were put on a train headed back to Moscow.

"This is the first time I know of that any of the tourist) groups were picked up by (the Soviets)," said Irene Manekofsky, president of the Washington Committee for Soviet Jewry.

"We were told we were 'bad tourists,'" said Marcia Weinberg, laughing, of the questioning incident. "They told us foreigners were not allowed in that part of the woods." She and Dr. Oscar Dodek, a gynecology professor at Georgie Washington University, and his wife Joan, said they were given a "stern lecture" for traveling without their group before they were sent back to their Moscow hotel.

The group, who brought medicines, blues jeans and other American clothes, and trinkets for those in prison to barter with guards, are not discouraged by the experience. Many of them see it as an "adventure," even a lark.

But to many Soviet and American officials the "refusednik" community aboard and their allied in the United States constitute another source of friction between the two countries.

Those who have been refused emigration permission often are the object of Soviet displeasure. Many have lost their jobs and some are in jail. Manekofsky said the 3,000-plus community is "virtually supported" by gifts from foreigners such as the Weinberg group. Blue jeans bring up to $100 in the Soviet Union, she said.

Many of those featured in the Weinbergs' slide show are relatives and associates of Jews labeled "dissidents," some of whose names are well known to readers of the American press.

Four of the Americans who attended the picnic were not arrested, among them Rabbi Weinberg. The photos he took there included Jewish rights activisit Alexander Slepak, an electronics engineer, and his wife Masha, a radiologist who were arrested shortly after the group left for home early this month. They face trial for "malicious hooliganism." Manekofsky called the Slepaks' "the most serious case in the Soviet Union. We are witnessing the destruction of this family."

Weinberg said the picnic was "ringed around with KGB men." The seriousness of the situation was driven home, said Phyllis Santer, a nurse at Jefferson Memorial Hospital in Alexandria, when she realized that officials watching the picnic seemed to take an interest in the group she was laughing with.

"I looked up and realized then that they could take any of us away," Santer said. She realized, too, she said, that the Soviet Jews at the picnic knew of that possibility. "They really should have told us," she said. Weinberg said they were advised to speak Hebrew or Yiddish and speculated, along with his wife, that she and the Dodeks were picked up because they were speaking English.

He said the picnics, held about every two weeks, are the community's substitute for synagogues, which he said are part of Soviet officialdom. "Many American Jews don't realize that the gifts they bring to ths synagogues end up being given to the officials. That's the price of being a rabbi in the Soviet Union."

The Weinbergs and others in the group laughed over ways in which they foiled what they believed to be Soviet attempts to keep them under surveillance.

"We never talked in our hotel room. We wrote notes and tore them up and put them down the toilet," said Rabbi Weinberg, who had to train many in group to call him "Joe," instead of "Rabbi." He listed his occupation as "teacher," a common practice for American clergy visiting the Soviet Union.

"One time," he said laughing, "we burned up some of the notes we wrote. It made so much smoke, we set off the (ceiling) sprinklers" in their hotel room.

A State Department source said that such tourists are troublesome because "they deliberately make a nuisance of themselves to Soviet officials when they go there . . . A lot of them think it's a game. But it's not. It's a very complicated sitution."

Santer doesn't think what she did in the Soviet Union was a game. "We went there to make contact. They were so alive and vibrant. They said they are the only free people in Russia. They've stopped listening to the Soviet line."

Santer said she is of Eastern European background and described the Jews she met as new-found "family." "If we were born in Russia," said Joan Dodek, "we'd be 'refusedniks.'"

"That is, if we'd survived," added Santer.