The Soviet Union indirectly accused the United States today of indulging in "futile propaganda" at the Belgrade conference on East-West detente.

It was the first reply here by the Soviets to American criticism of their human rights record and marked a distinct shift away from their initial low-key reaction. The conference is reviewing implementation of the 1975 Helsinki declaration on European security and human rights.

Speaking at a closed plenary session, chief Soviet delegate Yuli Vorontsov reportedly warned of the danger of turning the Belgrade meeting into "an arena of psychological warfare." He said this would distort the task "set before the meeting by the heads of state and government who signed the final act in Helsinki," according to informed sources.

Without mentioning any names, Vorontsov reportedly first praised a majority of delegates for speeches aimed at strengthening peace and cooperation in Europe. He then added, according to Western delegates who were present: "Against this general background, individual verbal excursions of a propagandistic character, devoid of any constructive elements, sound even more discordant."

Delegates said his remarks were clearly aimed at the U.S. delegation, which has gone further than any other in singling out specific cases of alleged human rights violations in Eastern Europe. Yesterday U.S. delegate Joyce Hughes accused the Soviet Union and Czechoslovakia by name of censoring mail in violation of international postal agreements.

Vorontsov's attack was immediately rejected by the United States, which said in a statement that it neither sought a confrontation in Belgrade, nor expected one. "The Soviet delegation is opposed to polemics. So are we, and there have been none," the statement said.

It was not immediately clear whether "the Soviet counterattack would lead to a modification of American tactics over the coming weeks. It came while Ambassador Arthur Goldberg, the chief U.S. delegate, was preparing a speech directed to human itarian problems that he is to deliver Thursday or Friday.

Vorontsov also indirectly criticized the 1974 U.S. Trade Act that bars the granting of most-favored-nation status to countries that do not allow free emigration.

A U.S. spokesman later said that the need for such legislation, inspired mainly by Soviet restrictions on Jewish emigration, depended in large measure on the extent to which the Helsinki declaration was fully implemented.