Even before the Soviet Union charged Jewish activist Anatoly Scharansky with high treason, Washington Kremlinologists privately warned that a formidable new effort against Russian dissidents was immient despite the forthcoming Belgrade review of the Helsinki agreement on human rights.

Indeed, no less a figure than exiled Nobel laureate Alexander Solzhenitsyn, who has given $300,000 to help Russian dissidents and their families, has signalled friends here from his Vermont retreat. Solzhenitsyn's warning: The KGB (secret police), stung by inability to control political dissidents, is preparing to "stamp out the movement" in a sweeping "legal" attack that would transform clearly political offenses - many of them permitted by presumed Helsinki human-rights guarantees - into synthetic "criminal" offenses.

Almost prophetically, Solzhensitsyn's warning anticipated the espionage charge against Scharansky, who has been in jail since mid-March.

How far Soviet authorities choose to carry their campaign against dissidents may well depend on how hard the Untied States insists that human rights must have a prominent place on the Belgrade agenda. This question will not be answered until diplomats form the United States, Canada and European countries that signed the 1975 Helsinki agreements start the preliminary post-Helsinki session in Belgrade on June 15. That session is supposed to set the agenda for the October meeting.

If the United States - possibly opposed by some Western European allies - insits on full exposure and debate over human rights, the Kremlin may have to tone down its new assault on dissidents. But if President Carter continues to play down the issue of human rights in the Soviet Union as he has done for the past two months, then Moscow may feel politically safe in trying to liquidate the humiliating agitation for civil and human rights.

The way that may be done is now becoming clear: by attacking the dissidents for "criminal" offenses and hailing them into public show trials on such charges as currency manipulation or treason or "hooliganism" or a variety of others, none tied to human rights.

There are today two principal targets of official attack against Soviet citizens: dissidents publicly working inside the Soviet Union as members of the "Helsinki monitoring group" and dissidents working with Solzhenitsyn's "Russian social fund," the $300,000 in royalties from his "Gulag Archipelago" that he has slipped into the Soviet Union to help dissidents and their families.

Scharansky was a leading activist in the Helsinki mointoring group when he was arrested last spring. At least eight other dissidents striving to publicize Soviet violations of the human rights stipulated at Helsinki - such as the right to emigrate - are also in jail. They include dissident leader Alexander Ginzburg, who was also the first head of Solzhenitsyn's fund. Arrested in early February, he has been in jail incommunicado and without formal charge.

Poignant testimony of the hopelessness and fear of a new Soviet clamp-down among families of jailed dissidents came in a letter just received by Solzhenitsyn from Mrs. Ginzburg and the wife of Yuri Orlov, another Helsinki monitor also arrested and jailed. The letter stated their "profound anxiety and preoccupation as to the fate of our arrested husbands," and went on:

"Ominous rumors have been spreading around Moscow. According to these rumors, (Ginsburg and Orlov) will be tried on the charge of having violated regulations on currency . . . We greatly fear that the authorities have decided to stage a criminal rather than a political trial. There has been a press campaign pointed in that direction."

The Soviet refusal to give a visa to Edward Bennett Williams, the noted criminal lawyer who has intimate ties to President Carter's Democratic Party, is further evidence of how hard-nosed the Soviets have become about their human-rights vulnerability. Retained by Solzhenitsyn to go to Moscow for the legal defense of Ginzburg, Williams has been unable even to contact Mrs. Ginzburg since the Soviets jammed their Washington-to-Moscow telephone conversation on March 31.

Clearly, the Kremlin is increasingly worried, partly about the continuing agitation of Soviet citizens trying to claim their Helsinki rights, but far more about the impact of the dissidents on the Belgrade conference. How President Carter will exploit this open wound is not yet known. But if he fails to make the most of it he will, at the very least, be open to severe criticism by many American politicians - and just plain voters.