The Soviet Union tonight outlined its formal position at the nuclear talks with the United States in Geneva, calling for staged reductions of medium-range missiles in Europe to "300 units on each side" by 1991.

In an authoritative statement distributed by the government news agency Tass, Moscow rejected proposals advanced by President Reagan last week as "patently unacceptable." It said they amounted to a demand for unilateral disarmament by the Soviet Union.

Tonight's statement provided a formal, detailed explanation of a suggestion made by President Leonid Brezhnev last week that the two sides should reduce their medium-range nuclear arsenals by two-thirds in the course of the decade. It also indicated that no major progress has been achieved in the Geneva meetings.

With tonight's statement, the Soviet presentation of its initiative appeared to parallel the U.S. approach. Reagan first outlined his "zero option" proposal and then publicized the formal U.S. bargaining position a week ago.

Reagan's zero option calls on the Soviets to dismantle their SS20 missiles as well as older SS4s and SS5s directed at Western Europe. In return, the United States would abandon the plan to begin deploying 572 medium-range nuclear weapons in Western Europe next year.

The Soviets, who insist there is already an essential equivalence in medium-range weapons, propose to talk about reductions by 1985 of two-fifths of the respective arsenals, to be further cut by 50 percent in a subsequent stage.

Saying that at present there are "approximately 1,000 units on each side," the statement calls for negotiators in Geneva to establish "an intermediate level of 600 units by the end of 1985" and reach an agreement for a reduction "to 300 units on each side toward the close of 1990."

It said "the main means of reduction of medium-range armaments will be their destruction, which does not exclude the possibility of withdrawing a part of the armaments behind some agreed lines."

The statement in Tass also called for provisions to be worked out for "adequate control" to assure compliance with terms of the anticipated accord.

The Soviets also proposed that "a framework" be negotiated for replacement and modernization of the remaining weapons. The Soviet proposal for cutting back medium-range nuclear arms in Europe also included attacks on the United States, whose "imperial arrogance" was said to be blocking progress in Geneva.

In making the details public, the Soviets appeared to be making a fresh bid for public opinion in Western Europe, where there has been strong opposition to the NATO plan to deploy a new generation of medium-range, U.S. nuclear missiles there.

The basic difference between the two sides appears to be their respective assessments of the current nuclear balance in Europe. The Reagan administration argued that the Soviets have gained superiority in medium-range nuclear strength and therefore should remove their rockets. Moscow insists that U.S. forward-based planes and submarines as well as British and French nuclear weapons must be included in the calculations--to show both sides with nearly equal numbers of delivery vehicles.

The "substance of [Reagan's] proposal lies in the fact that the Soviet Union would be required to liquidate unilaterally all its medium-range missiles, not only in the European part of the Soviet Union but also those deployed in the eastern areas of our country, that have nothing whatsoever in common with the problem of nuclear weapons in Europe," the statement said.

This would mean in effect that NATO would gain "a more than double advantage regarding the number" of delivery systems and "triple" advantage in the number of nuclear warheads, said the Soviet statement.

"Washington's real design, as can be clearly seen, lies in creating a stalemate at the talks by tabling proposals patently unacceptable to the other side to block the [talks] and then to try to lay the blame for this at the Soviet Union's door."

The statement said that U.S. negotiators in Geneva have so far only marked time by repeatedly advancing Reagan's "unrealistic" proposals.

Tonight's statement was notable for its strong insistence that the nuclear systems of the British and French be included in the Geneva talks. "It is not a question of their participating in the negotiations or signing an agreement, since they do not want this," the statement said. "But it is clear that their respective armaments must be regarded as a component of the balance on the NATO side."

The Soviets also renewed their charges that Washington was using the Geneva talks to "calm the West European public opinion protesting against the dangerous military plans of the United States and then, by deliberately leading the negotiations into a blind alley, try to justify in this manner the deployment in Western Europe beginning in 1983 of almost 600 new U.S. medium-range missiles."

"It looks as if the propaganda show baptized as the so-called zero option was needed precisely for this," Tass said, calling the prospective deployment "one of the avenues leading to the creation of a first-strike capability against the Soviet Union."