President Leonid Brezhnev appears to have tried to seize the diplomatic initiative in Europe through an interview with a West German magazine last week that, judging by Soviet commentaries, has been greeted by paeans of admiration not only here but throughout Western Europe.

The Soviet leader for the first time publicly talked about details of Soviet arms strength, including the numbers of medium-range nuclear rockets aimed at the region.

Brezhnev gave Der Spiegel his view of East-West relations in conciliatory form. He spoke almost sadly about Washington's defense policies as being responsible for the current tensions and renewed Soviet offers to negotiate.

Taking personal charge of efforts to court West European opinion, Brezhnev suggested that the new U.S. leaders were cynical men prepared to allow technological developments in nuclear arms to "get ahead" of public desire to control such weapons.

The fact that the full text of his interview was published in Pravda and all other newspapers -- and given the most prominent front-page display -- makes his statements the most authoritative expression of Soviet thinking with the approach of U.S.-Soviet talks scheduled later this month on limiting theater nuclear weapons.

The focus of his interest was clearly Western Europe, where he will travel in two weeks for an official visit to Bonn.

The main issue worrying the Soviets is the scheduled deployment in 1983 of 572 U.S. medium-range nuclear missiles in Western Europe. Although Chancellor Helmut Schmidt has been one of the main backers of deploying such weapons to counterbalance Soviet forces, the issue has turned into a divisive political dispute, particularly in the Netherlands, Belgium and West Germany.

President Reagan's recent remarks about the possibility of a limited nuclear exchange have only increased European anxieties. The Soviets, in turn, have sought to foster the notion that Europe might become an unwilling victim of a nuclear exchange.

Brezhnev sought to exploit U.S.-European differences, apparently trying to convince the Europeans that the NATO deployment would not give them any military gains but would increase military and political risks.

The main thrust of his argument was that the new mobile SS20 Soviet missile is indeed an advanced weapon but does not represent a new threat. In contrast, the proposed deployment of the U.S. missiles is an new threat to the Soviet Union, he argued.

At the moment, he insisted, there exists a "rough balance" between the Soviet Union and the United States and other NATO countries in medium-range weapons.

The Soviet Union, he continued, has 975 such weapons, compared to NATO's 986. Britain had 64 missiles capable of reaching targets in the Soviet Union and 55 medium-range bombers, France 144 such units and the United States more than 700, including F111, FB111 and F4 aircraft and submarine-based launchers.

That count is disputed by Western military analysts, who point out that while Brezhnev counts U.S., British, and French bombers and submarines with nuclear launchers, he does not count Soviet bombers and submarines capable of striking Western Europe.

But Brezhnev argued that the "service life" of Soviet SS4 and SS5 missiles has "expired" and that "when deploying one new SS20 missile we replace one or two old missiles and scrap the latter together with launchers."

"It is true that SS20 can carry three warheads, but their combined yield is less than that of one old warhead. Consequently, in the process of replacement of obsolete missiles, the total number of our carriers was decreasing and simultaneously the aggregate yield of our medium-range potential was reduced."

He rejected as "far-fetched" charges about Soviet superiority and said that if U.S. medium-range rockets were deployed in Western Europe, NATO would gain a "50 percent advantage" in the number of nuclear launchers and "almost a 100 percent advantage" in nuclear charges.

The Soviet leader's message to Western Europe was that this strategy carries "vast danger" for the region, which the United States wants to turn into a "launching pad for new missiles trained at the Soviet Union." This, he added, is a part of U.S. efforts to gain strategic superiority.

"Everything in the official explanations" coming from Washington about the need to deploy the new rockets "is false from the beginning to the end." Remember, he said, how the United States reacted when Moscow attempted to place rockets in Cuba.

He said that "in order to neutralize mobile missiles" the Soviets would have to make retaliatory strikes "of great yield" at the areas where they are deployed.

But he said that the Soviet Union "will under no circumstances" use nuclear weapons against countries that do not have such weapons deployed on their territory.