With just a few weeks before the opening in Belgrade of what may be a major east-west debate on human rights, the Kremlin - through arrests, threats and deportation - has all but throttled the tiny, vocal group of human-rights activists whose inquiries and reports could form an important part of the allegations that may be discussed at the conference.

While achieving their goal of virtually silencing critics, the Soviet leaders have:

Sharply criticized President Carter's public shows of support for the dissidents.

Warned their East European allies about the dangers of dissidence.

Told the rising Communist parties in the West that the new Soviet constitution, now pending, shows individual freedoms to be at an alltime high.

"As a damage-limitation policy, they've succeeded," commented one well-informed Western diplomatic observer. "They have nearly broken the Jink between the dissidents and the Western press . . . There is a lack right now of dissidents figures who can attract questions from the West."

Concern among diplomats here is shifting to what many say will be the next phase of the repression of dissenters: a public show trial of a jailed human-rights activist.

Recent history shows the Kremlin capable of disregarding outcries in the West and proceeding efficiently at stoppering dissent. Of 10 original members of citizens' group formed last year to monitor Soviet compliance with the human-rights provisions of the Helsinki agreement on European security, four have been arrested, three sent abroad, and one is in Siberian exile. Two others, including Elena Bonner, wife of Andrei Sakharov, have not been subject to formal action.

Two Ukrainian activists have been tried, convicted and sentenced to long terms at hard labor for their attempts to file reports about human-rights troubles there.

Numerous others who were active in dissident circles have been given exit visas to Israel whether they initially sought them or not. One in that catergory is Valentin F. Turchin, 46, a computer expert who was given permission to leave last week. Earlier this year, Turchin said he did not choose to leave the Soviet Union.Since then, he has been subjected to questioning, threats and isolation and has had to consider what becomes the final, grim choice here so many like him: Leave the country or go to jail.

As the summer waned, Western diplomats speculated on the future of a possible trial of a dissident, with some guessing it could take place well in advance of the Belgrade reviews of the Helsinki accords so the Soviets could, in the words of one source, "get things cleaned up in time for Belgrade."

Now, most informed observers here think any trial could be put off until after Belgrade, giving the Soviets time to bargain whatever they can at the conference without having to worry about the possible consequences of a blitz of bad publicity which could be sparked by any trial.

The United States and Soviet Union are clearly heading for a clash in Belgrade on the issue of human rights, with each side accusing the other of gross violations of the so-called "basket three" provisions of the agreement that was signed by 35 nations in Helsinki in 1975. That accord sets forth basic principles of European security and cooperation and includes provisions in its third segment, or "basket," for freedom from political imprisonment, and easing the bars to emigration and reunification of families.

Foremost in the minds of most Western sources here when talk turns to a possible trial is Anatoly Scharansky, who reportedly faces charges of treason, a capital crime. Scharansky, a young computer technician, was a member of the Moscow group that sought to monitor Soviet compliance with the Helsinki agreement. He signed several of the nearly two dozen reports they issued during 1976 and early 1977 on alleged abuses of individual freedoms.

Scharansky, a Jew, isis a "refusednik," a person who has been refused permission to emigrate. As such, he was a member of a community of Soviet citizens scorned and vilified by officialdom and almost invariably sacked from their jobs within a short time of applying to emigrate.

The two other prominent jailed dissidents are Yuri Orlov, founder of the Moscow branch of the Helsinki monitoring group, and Alexander Ginzburg, who was a monitor and also administrator of a fund for political prisoners and their families financed by royalties from the works of Alexander Solzhenitsyn, the expelled Nobel laureate writer.

The causes of the three men has been widely publicized in the West and their plight taken up by various groups in America and Western Europe. One source here speculated that "if they put Scharansky on trial, the American Jewish groups are truly going to raise hell. They are a real political force and have clout in the U.S. They can pressure American firms not to trade with the Soviets, demonstrate at Soviet fairs and embassies, and there may even be new legislation in the Congress that would be a rebuff for staging a Scharansky trial."

The Soviets have tried to paint all the dissenters as akin to common criminals and, by doing so, to clear the way for whatever fate the state may find their due. The head of the KGB, the Soviet secret police, recently spent about a third of a lengthy televised speech denouncing dissidents as "an isolated band of renegades." Yuri Andrepov described them as "a few people who have cut themselves off from our society . . . on a course of anti-Soviet activity," who "violate our laws, supply the West with slanderous information spread false rumors and try to organize various anti-social provocations."

He said the dissidents exist "only because the enemies of socialism brought into the business the Western press, diplomatic and intelligence as well as other special services," he said, adding that dissidents are "generously paid" in hard currency.

Andropov also said that there are fewer persons in prison now for anti Soviet activities than at any time since the 1917 revolution, a statement clearly intended for the ears of West European Communists, some of whom have criticized the Soviet Union for its handling of the dissidents.

The KGB chief's speech was followed in a few days by a series of unusual scenes at the Moscow international book fair, where a number of dissidents writers appeared daily, including Vladimir Voinovich and lev Kopelev Voinovich later told reporters that he had been subjected to close "tailing" by several security men. Western sources commented that the apparent free travels of the writers was "an important ingredient to the Soviets in their attempt to look good at Belgrade," as one source put it. "They know the American publishers are deeply committed to questions of freedom to publish, and that the Americans also have things . . . the Soviets want to do business with them and so they were careful not to interfere. But it signals no real change in attitudes at the top."

As for the future of the dissident community, a person who has observed it for some time and has close ties to it said recently, "There are a number of people who want no contact with Western diplomats or reporters. They are part of a secondary group that lends support to the public dissidents, but is unknown to Westerners. they are falling back a little, just now, but they are there. It may be that in time, they will be heard from themselves."