The 35-nation preparatory conference that is to open here Wednesday could explode into a bitter new clash between the Kremlin and the Carter administration over human rights, or could prove to be just what was needed to calm growing tension between the superpowers.

Whichever course the conference eventually takes, the proceedings here will put the future of East-West relations to a severe test and the outcome will affect, to one way or another, the lives of millions of central Europeans caught in the Washington-Moscow crossfire.

Delegates are gathering here from 33 European countries, the United States and Canada to review progress, or lack of it, in implementing the Helsinki agreements on European security and cooperation that their governments all signed in August 1975.

In part, those accords pledged all signers to respect "human rights . . . fundamental freedoms . . . and the freer movement" of both people and ideas.

It is that provision, more than any other, that has sparked unrest by dissidents throughout Eastern Europe and charged the political atmosphere with more tension than either Washington or Moscow anticipated almost two years ago.

The review conference thus opens at a time of dangerous and emotional big-power cross-purposes and amid fears by many Europeans that a U.S.-Soviet confrontation here could put an end completely to the hopeful "spirit of Helsinki."

The approach of the Belgrade conference itself has contributed to the very tensions that the Helsinki accords were supposed to ease. Last week the White House issued a 93-page document reporting Soviet implementation of human-rights pledges as disappointing; meanwhile, the Soviets are collecting ammunition about everything from racial discrimination in the United States to torture in Northern Ireland to hurl back at the West.

Indeed, one of things delegates will have to agree on before they go home is whether there will be another review conference.

The Belgrade meeting is specifically called for in the Helsinki agreement. Beyond, that, nothing is set. There is some fear that if the Soviets and their European allies are publicly embarrassed by an American-led attack on their human-rights policies, the Soviets will not agree to another review conference where the same thing could happen again.

At Belgrade, all major decisions will be by consensus, and while few Western diplomats believe that the Soviets would go so far as to let the review process die, "it is conceivable," cautions one senior diplomat.

Wednesday's opening will not mark the start of actual discussions on how well Helsinki has been observed.

Rather, this preparatory conference is expected to run three to six weeks and is meant only to set up the agenda, duration and procedures for handling the main substantive review of the Helsinki accords. That is likely to begin here the first week in October.

This is what one Westerner call the "dull but tricky" phase.

Despite the purely organizational nature of these first few weeks, the battles over procedure are viewed as crucial to achieving the "constructive" main conferencem that all countries claim to want.

For example, because of the ever-present danger of confrontation and collapse, Western officials stress that "the way" in which delegates at the main meeting can make their points is critical.

The West prefers to maintain the form used in the Geneva conferences of the early 1970s that led to the Helsinki accords. This means individual working committees fo each of the agreement's three parts, or so-called baskets: military security, economic and cultural cooperation, and human rights.

This allows more detailed reviews in each area. But the soviets this time are viewed as likely to object, because of the sensitivity of the human-rights issue, and propose instead that business be conducted at full plenary sessions where participants discuss their own efforts without cross-examination.

Linked to this general concern are Western European fears that statements made publicly outside the closed conference rooms could influence the course of the meeting.

The Western Germans, for example, are worried that congressmen in the 15-member U.S. delegation may not be able to resist coming out of a secret working committee and telling newsmen, "We gave them hell to there."

"We have lots of points to grill the Russians with. But it must be done in clever and concerted action," a Bonn diplomat said, "where public pressure and diplomatic action are clearly kept apart. We can't have overwhelming public discussion and humiliation of the Soviets that makes diplomatic action impossible."

Similar worries among Europeans are directed at President Carter. To many of them he appears unpredictable and emotionally involved in the human-rights question.

Europeans here have strong praise for the firm yet moderate tone of Secretary of State Cyrus Vance's recent statement on the U.S. approach to Belgrade. But Europeans on a divided continent - affected more diretcly by barriers to movement and dependant on flourishing East-West trade - feel that they have far more to lose than Americans if the conference collapses. For them, the prospect that Carter may unleash another barrage aimed at Moscow during the conference is unsettling.

On the other hand, American and Western European sources here say they have the feeling that Carter has made his human-rights position abundantly clear and is apt to "keep his powder dry" now and let Belgrade take its course.

Thus some of these diplomats feel that the meeting here may turn out to be a good forum for cooling off the blistering anti-Carter rhetoric that has spilled out of Moscow in recent weeks.

Few international conferences in the post-World War II era have actually taken place amid so many emotional issues that must be controlled so as not to wreck the meeting at the outset, or amid so many historical ironies.

In some ways, the events of the past two years have made the Belgrade review more important politically than the Helsinki agreements.

When those accords were signed, they were widely perceived in official U.S. circles as largely a propaganda victory for the Soviets because parts of the documents amounted to Western ratification of the Soviet Union's postwar territorial dominance of Eastern Europe.

There was little real interest in Helsinki in Washington and a relatively low-level U.S. delegation was sent to the Geneva preparatory talks. President Ford, sources say, had relatively little feel for the agreements and Secretary of State Henry A. Kissinger once joked of them as "a basket case." Even Carter, during his election campaign, condemned those parts dealing with Eastern Europe.

But the Soviets also misjudged the long-term impact of those accords.

The human-rights sections were inserted at the insistence of the West and agreed to by the Soviets as a compensation for the presumed Soviet advantages in the provisions dealing with military security and economic and cultural cooperation. The humar rights sections now have turned out to be a huge political embarrassment for Soviet Party leader Leonid Brezhnev, who was the basic instigator for a Helsinki agreement.

These provisions have also provided a natural local point for a new American President who terms his committment to human rights "absolute."

Throughout much of Eastern Europe - in Poland, the Soviet Union, Romania, East Germany and Czechoslovakia - thousands of people have sought to hold their Communist regimes to their promise by demanding the rights pledged, perhaps toe casually, at Helsinki.

The United States has increased the clout of its team at Belgrade by naming deputy Secretary of State Warren Christopher, the No. 2 official at the State Department, to head the delegation for the main meeting. The Soviets, apparently ready for a big-power fight, have named their No. 2 man in Washington for many years, Yull Vorontsov, to head the Soviet delegation here.

The dilemma for the West is how far the Soviets can be pushed without breaking down the process of relaxed tensions, free emigration and reunification of divided families that the accords were supposed to promote.

There have always been two schools of thought on this in the West.

The first sees some seeds of moderation in the Soviet leadership, particularly in Brezhnev, and argues that a policy of moderation in dealing with the Soviets will allow those seeds to grow.

The other views, the Soviets as unable or unwilling to yield anything except under strong external pressure.

Which one will emerge over the next several months here is something nobody seems to know.