Fundamental differences emerged here today in U.S. and Soviet approaches to reviewing how well nations have carried out the far-reaching pledges on European security, economic cooperation and human rights signed by 35 countries at Helsinki in 1975.

The differences became clear in the second day of the 35-nation preparatory conference meant to lay the groundwork for a full-scale Helsinki review this fall.

In the first Soviet address to the conference,chief delegate Yuli Vorontsov called for "a constructive atmosphere and businesslike exchange of views." He said the emphasis should be on "summarizing the positive accomplishments" since the accords were signed and in "looking toward the future" with respect to new proposals.

In the way Vorontsov said, the conference would "contribute to the general improvement of the political climate."

In effect, the Soviets are asking the participants not to criticize one another or examine in detail individual countries' failure to live up to the spirit of some of the pledges made at Helsinki.

Those pledges, however, inspired an upsurge of demands for individual freedoms and human rights throughout Eastern Europe, and the Carter administration has undertaken a major campaign of pressing the Soviets on this issue and openly criticizing them for a "disappointing" performance in the two years since Helsinki.

The U.S. delegation here is arguing for the complete review of implementation of all parts of the accords that is called for in the final act of the Helsinki agreements.

The United States, backed by most West European countries, is pressing for an open-ended debate here next fall that will not end until all points are examined and that will include working committees as well as full conference groups so detailed assessments can be made.

A proposal introduced yesterday by the British, with the support of the European Common Market and the United States, would set up that kind of open-ended agends, although it did include a provision for a non-binding goal of ending in 12 weeks.

Today, however, the conference seemed to be turning instead to a simplified Spanish proposal that lays out an agenda of questions for the preparatory meeting to decide without suggesting any specifics.

Thus, it now seems that this conference will enter a long period of perhaps six weeks in which each point about duration, agenda and committees for the main conference will be argued out. United States would press for no specific closing date for the main conference and the Soviets would try to assure shutting things off before Christmas.

In Vorontsov's view, "If we are to speak about our major achievements" in Europe since Helsinki, "they are that peace in Europe has become more stable, that economic relations have expanded, that there are broader cultural and information exchanges and (that) contacts among peoples are developing."

The meetings here are all behind closed doors, but a spokesman for the Soviet delegation briefed reporters on Vorontsov's comments.

Although most of the West would disagree with much of Vorontsov's assessment, the U.S. delegation chief, Ambassador Albert w. Sherer, said later that he found the Soviet's opening speech "basically constructive." Asked earlier how the conference seemed to be going, Sherer replied, "Perhaps it's going better than I expected."

Privately, U.S. officials acknowledge the exchanges between Washington and Moscow over human rights has gotten so hot lately, along with Soviet arrests of dissidents and harassment of American journalists, that they had arrived hear fearing that the conference could collapse even on the opening day.

For the moment, there seems to be a conscious Western strategy to calm things down and avoid public clashes if possible, so the preparatory conference can come up with procedures for the main review similar to what the West proposed yesterday.