The Soviet Union tonight dismissed President Reagan's proposal on reducing nuclear forces in Europe as a "propaganda ploy designed to stalemate" the forthcoming Geneva talks on medium-range missiles.

In a quick rejoinder to Reagan's address earlier today, the official news agency Tass described as "absolutely fantastic" his figures on East-West military forces in Europe.

Tass also repeated the long-standing Soviet position that no positive results could be expected from the Nov. 30 Geneva talks if the United States refuses to negotiate on its forward-based systems -- that is, aircraft in Europe capable of carrying nuclear weapons -- and the British and French nuclear forces.

The Soviets already had ruled out the so-called "zero option" embraced today by Reagan. It calls for the Soviets to remove recently stationed SS20 medium-range rockets targeted on Western Europe in exchange for the United States canceling deployment of Pershing II and cruise missiles there.

This type of proposal is addressed in Moscow as posturing. Tass linked it to what it termed "extraordinary measures to provide propaganda backing" for Reagan's speech and to bring "maximum pressure on public opinion, primarily in Western Europe where the antiwar movement is gaining momentum." It noted that the U.S. government "paid for live transmission" of the speech to Western Europe.

But the tone of Soviet comments was cautious, perhaps reflecting an intention to cast doubts on what is seen here as a U.S. attempt to rally wavering West Europeans behind Reagan's defense policies.

Diplomats here said the Reagan speech was timed to precede Soviet President Leonid Brezhnev's scheduled visit this weekend to West Germany. In an interview with the West German weekly Der Spiegel two weeks ago, Brezhnev sought to exploit West European reservations about a new generation of medium-range missiles by insisting that the scheduled deployment would not produce military gains but would increase military and political risks.

Tass focused almost exclusively on the president's proposals dealing with medium-range nuclear weapons in Europe, ignoring other parts of his speech.

A main objective of Soviet policy has been to generate opposition in Western Europe to the scheduled deployment of new U.S. rockets, thus affecting the nuclear link between America and the rest of the NATO alliance.

Tass and a commentary due to appear in Friday's issue of the journal New Times both assert that Reagan's proposals suggest that the United States would like to see a breakdown of the Geneva talks "that could be used as an excuse for the continuation of the arms race."

"This is forcefully corroborated by the fact that the Americans actually declare in advance that their forward-based systems and the nuclear weapons of their NATO allies are not to be discussed at the Geneva talks," Tass said.

It summed up Reagan's proposal as "the elimination of the Soviet Union's existing defense potential in Europe while the American forward-based systems and the submarine-based missile complexes and nuclear bombers of Britain and France will be preserved."

The commentary restated figures given earlier by Brezhnev, who asserted that "rough parity" prevailed in Europe, with NATO having 986 medium-range nuclear delivery systems to 975 such systems in the Soviet Union.

Reagan's assertion of Soviet superiority in medium-range nuclear systems was described in these terms by Tass: "He cited absolutely fantastic figures showing the alignment of forces to be about six to one, contrary to incontrovertible facts that corroborate the existence of rough parity."

Tass said NATO's weapons of this kind include 700 U.S. aircraft, Britain's 64 ballistic missiles and 55 bombers, and France's 98 missiles and 46 bombers.

The agency described Reagan's speech as "unrealistic" and "intended for propaganda purposes." But the Soviets clearly welcome the Geneva talks, apparently on the assumption that as long as these negotiations are under way it would be difficult for the United States actually to deploy the 572 Pershing II and cruise missiles as scheduled in 1983.

Earlier today, U.S. Ambassador Arthur Hartmann visited Soviet Foreign Minister Andrei Gromyko to provide him with details of Reagan's speech along with his letter to Brezhnev.

A U.S. Embassy spokesman said Hartmann and Gromyko spent more than one hour discussing "substantive" issues. He declined to elaborate.

Brezhnev in his interview with Der Spiegel also exploited Reagan's public discussion of a possible limited nuclear war, statements that senior diplomats here say have caused considerable discomfort to several NATO governments. Reagan's speech today was seen as his answer to that Brezhnev approach to Western Europe.

What all of this boils down to, according to some diplomats, is that the Geneva talks are likely to be complicated and long.