In a 45-minute meeting with American peace activists and former presidential candidate Jesse Jackson, Soviet leader Mikhail Gorbachev today praised Soviet Jews while dismissing any concern over their situation, spoke warmly of the people of America and joked about Americans and Russians marrying.

The 54-year-old Soviet leader had just finished more than two hours of talks with President Reagan, and a joint U.S.-Soviet news blackout on those meetings had just been imposed.

Soviet television cameras zeroed in as he appeared in the lobby of the Soviet mission to the United Nations here to meet with Jackson and 49 other peace activists and receive a petition signed by more than 1 million Americans urging him and Reagan to sign a nuclear test ban as a prelude to a nuclear freeze.

The activists, all but one American, seemed stunned when Gorbachev actually showed up.

There had been only rumors that the unnamed Soviet official who would meet them would be the country's new leader.

Gorbachev and Jackson stood, barely three feet apart, in the center of a tight circle of activists, members of the press and Soviet security. The crowd had rushed forward to surround the pair from the horseshoe formation security officers had arranged.

In response to Jackson's request that he discuss "the plight of Soviet Jews," Gorbachev said, "Jews are a part of the Soviet people. They are fine people. They contribute a lot to disarmament. They are very talented people and they are very valued in the Soviet Union.

"The problem -- the so-called problem -- in the Soviet Union does not exist. Perhaps this problem only exists with those who would like to mar the relations with us, who cast their doubts and aspersions."

After the meeting ended, Jackson told reporters, "On the question of Soviet Jews, his answer was not adequate to us. He recognized no problem."

Earlier in the day, a small group of Jewish activists protesting Soviet treatment of Jewish citizens occupied the offices of the Soviet airline Aeroflot for about two hours before being removed by Swiss police.

Harsh Soviet restrictions on the emigration of Jews and other minorities were eased during the 1970s but were tightened up again as detente waned.

Visiting U.S. officials periodically have raised individual cases on a quiet basis in Moscow, with some success, and President Reagan made clear that emigration and other human rights issues were firmly on his summit agenda.

In what was regarded as a significant step to improve the summit atmosphere, Soviet officials last week resolved the cases of 13 Soviets who will be allowed to join their families or spouses in the United States.

Earlier, they allowed a dissident to leave -- who has since interrupted Soviet briefings here, publicly assailing the Soviet record on human rights -- and indicated that Yelena Bonner, wife of dissident physicist Andrei Sakharov, will be permitted to travel to the West for medical treatment.

On the subject of travel to the Soviet Union, Gorbachev repeated several times his interest in having Americans visit his country.

"Fifty thousand U.S. citizens come to the Soviet Union every year and they travel around the country and they see what is going on there," he said. "We are for open doors, and we welcome everyone, including the United States citizen, to visit the Soviet Union, get acquainted with the Soviet people, and even get married. Why not?"

When Americans and Soviets meet, Gorbachev said, "It is always an easy-going and sympathetic atmosphere. Perhaps this is because the Americans and the Soviets are very much alike. At any rate, we are big nations, and we are straightforward."

As the two men talked and listened to the translations, they kept their eyes on each other's faces. During the discussion, Jackson smiled frequently and Gorbachev gestured in emphasis, turning at times to Soviet Ambassador to the United States Anatoliy Dobrynin, Foreign Minister Eduard Shevardnaze and Georgi Arbatov, an adviser on the United States.

Both men were forceful at times, Gorbachev gesticulating when discussing fascism and Jackson repeating his question on Soviet Jewry when the Soviet leader failed to address it. The activists were responsive, laughing loudly at Gorbachev's joke about marriage, craning to get a view and snapping their own pictures throughout the session.

The group listened to Gorbachev discuss a range of subjects raised by Jackson.

Directed to a table covered with stacks of nuclear test ban petitions, Gorbachev slapped his hand down and asked, "How many hopes do you have here?"

"A million and a half," Jackson replied.

"If you consider for all those represented, you have friends and family members, then those are millions upon millions," Gorbachev said.

The meeting began at 12:50 p.m., after the first session of the summit. When asked by an American journalist how he liked Reagan, Gorbachev replied, "We had a very calm, businesslike, pleasant talk. It has just started, therefore I won't tell you anything."

The crowd laughed.

Gorbachev also spoke about "breakthroughs in science and technology" that he said might escalate the arms race.

Speaking to reporters later, Jackson said Gorbachev "made it clear that his business is disarmament."

The American group was composed of members of SANE, the Nuclear Freeze Campaign and the coalition called Women for a Meaningful Summit. The groups had requested meetings with the Soviet leader and Reagan; they were received at the American mission earlier in the day by a State Department official.

After Jackson and Gorbachev finished, activist Justine Merritt gave Gorbachev a present for his wife, an embroidered panel from the Ribbon, a 15-mile long string of panels depicting what the thousands of Americans who participated in the project would most regret losing in a nuclear war.

The unexpected meeting with one of the two world leaders was just the kind of off-chance encounter dreamed of by hundreds of people here for the summit. From "Star Wars" supporter Phyllis Schlafly and her entourage of about 30 conservative women to a group of Afghan refugees to a Miami gay-rights activist asking for Reagan and Gorbachev to invest more in AIDS research, lobbyists have come to Geneva. They have been drawn both by the presence of the two summit principals and, perhaps more important, by a sizable contingent from the world media.

Gorbachev finally wrapped his scarf around his neck and left at 1:35.

"We've made our challenge and our requests to both sides," said SANE Director David Cortwright.