Yuli Vorontsov, the Soviet delegate at the European Security Conference, threatened last week to "break up" the conference after Western countries pointedly attacked the Prague trial of Charter 77 dissidents. But despite rising Soviet embarrassment, a Soviet walkout is highly improbable.

The reason is that the Belgrade conference, permeated with political surrealism and irony, is "Brezhnev's baby" - the fruit of Soviet leader Leonid Brezhnev's crowning achievement. That achievement, now producing very bitter fruit indeed for the Russians, was the European Security and Cooperation Agreement (ESCA) signed at Helsinki in 1975. The follow-up meeting here of European states, plus the United States and Canada, is supposed to be examining how well or badly the Helsinki agreements are being carried out-including the human-rights quarantees.

Although held under Brezhnev's imprimatur, the follow-up conference has become a unique and, for the West, rewarding demonstration of Soviet self-torture. Unable to walk out when the West politely touches the exposed nerve of Soviet human rights (the diplomacy here is vintage Congress of Vienna), Vorontsov mumbles feeble threats of a break-up. The real threats are coming from Moscow where a new trial may be cooking for Anatoli Shcharansky.

Strong hints of a Shcharansky trial, following the Prague trial of Charter 77 dissidents, seem to be calculated for their blackmail effect on this conference.

Thus, the Kremlin is saying that either the West (mainly the United States) shuts up about human rights or Shcharansky is doomed in a spy-trial linking him to American journalists. Shcharansky's sin, like the sins of the Charter 77 dissidents and of jailed Soviet citizens Yuri Orlov and Alexander Ginzbury, was to use a provision of the Helsinki agreement to try to monitor Soviet performance on the human-rights pledges it made at helsinki.

These pledges, signed by Brezhnev himself, "confirm the right of the individual to know and act upon his rights" - which is exactly what the dissidents tried to do when they established their Helsinki "monitoring" groups.

In the teeth of that Helsinki guarantee, Vorontsov has become a ridiculous figure crying "intervention" in Soviet internal affairs whenever Arthur Goldberg, the loquacious U.S. delegate, or another Westerner tries to get down to the work of the conference: to review the "implementation" of the Helsinki pact, including human rights.

"Day after Day Vorontsov has to sit there before all Europe and look ridiculous," one Western delegate of the Soviet Union hanging on its own rope is not entirely unpleasant even to such Eastern European states as Hungary, Poland and Romania.

After Goldberg (derisively called "the judge" by Soviet diplomats her) castigated the Soviet claim of "intervention" as "a complet distortion of the letter and spirit" of the Helsinki agreements, Romanian delegate Valentin Lipatti conspicuously strode over to shake hands and congratulate him. There are other examples of restrained Eastern European glee at Soviet discomfiture.

Unmistable Soviet pressure tactics, a trademark of the Kremlin's workaday diplomacy, go beyond hints of a brutal public trial for Shcharansky. When the West German delegate raised polite questions about Soviet Helsinki obligations to repatriate tens of thousands of, Volga Germans, Vorontsov replied that if the German "thinks that by intervening into Soviet internal affairs he will in any way improve the solution of bilateral matters, he is badly mistaken." Translation: Watch it, buddy.

Unable to deal with human rights, Vorontsov is falling back on subterfuges to "clog up the machinery," as one American puts it. Adroitly dealt with so far by Goldberg, these include Soviet proposals barring new members of military pacts (aimed at Spain joining NATO), bringing the long-stalled question of mutual arms of reduction under the Helsinki blanket, reducing the size of forces permitted in troop maneuvers, another anti-NATO gambit, inviting the Palestine Liberation Organization to address this conference.

"They are reduced to a damage-limiting defensive game here," one Western diplomat told us. "The Soviets have seldom been caught in such a public bind."

There is another Soviet game being played here: to break Western unity by splitting off the more reserved Western Europeans from the United States-the subject of another report.