With a blitz of letters from president Leonid Brezhnev to Western Europe's leaders, the Soviet Union has embarked on a campaign to win support for its foreign policy goals and, in the view of some diplomats. counter the Reagan administration's effect to move closer to its European allies.

As more Brezhnev letters were delivered today, the Kremlin's leading U.S. expert, George Arbatov, cautioned in Pravda that "all nations are faced with the necessity to make a choice, to define clearly their stand on the most burning, important issues of the day."

In addition to the lengthy letters, which explain the foreign policy outlined by Brezhenv to the Soviet Communist Party congress two weeks ago, Soviet officials are taking their campaign to the public, including appearances of top Soviet Embassy aides on U.S. television interview programs the past two Sundays.

The delivery of the letters during the weekend and today is being taken by Western diplomats partly as a reflection of genuine Soviet interest in resuming negotiations and avoiding an arms race with the United States. But it is also regarded as a mischievous, cleverly timed gambit to generate static in the waves of communication that President Reagan is trying to establish with America's traditional friends abroad.

The letters, sent to the leaders of most of the major countries in the North Atlantic Treaty Organization and to some neutral European states, are said by Western officials to elaborate on Brezhnev's appeal for a summit meeting with Reagan and his call for renewed arms talks, plus other proposals advanced at the party congress.

West Germany, France, Britain, Italy, Norway, Denmark and Sweden are among the governments reported to have received the letters. British Prime Minister Margaret Thatcher received hers in a meeting today with Soviet Ambassador Viktor Popov, British officials said, and told Popov that "it would be much easier to negotiate with the Russians if their troops were out of Afghanistan."

Delivery of the letter coincides with the visit to Washington by West Germany Foreign Minister Hans-Dietrich Genscher, and its principal target is assumed to be West Germany, where cooperative relations with Moscow remains a central tenet of government policy.

Although traditionally a most reliable ally in NATO, the Bonn coalition under Chancellor Helmut Schmidt has been unsettled by a swelling wave of pacifist and antinuclear sentiment directed against the 1979 NATO decision to place medium-range nuclear missiles in Western Europe.

There is also a strong movement against deployment in the Netherlands, where a decision is scheduled for December following elections in May. In neighboring Belgium, pacifists similarly are picking up force.

The restless state of Western European affairs has given Moscow ample field for diplomatic play. Soviet analysts see a calculated bid by the Kremlin to blunt the Reagan administration's initial strategy of rallying allied support for harder line against Moscow.

In what the French paper Le Monde today described as the "most classic method" of diplomatic attack, Soviet Officials have gone on the political offensive in Western Europe, fanning out in the aftermath of their party congress to explain Brezhnev's "signal" of readiness for negotiations.

Brezhnev's offer had tacked to it a call for a mutual East-West moratorium on the deployment of new missiles in the European theater.

Such a freeze was seen as unacceptable by Western governments, which recognized that it would preserve Soviet military superiority in Europe.

The NATO decision to station 572 U.S.-made cruise and Pershing missiles in Europe had been intended to restore the balance that was upset when the Soviet started deploying SS20 missiles.

In the 14 months since NATO approved its deployment, the Soviets have accelerated theirs, according to Western intelligence officials.

Bonn officials, however, credit Moscow with a shrewd tactical move. By suggesting a moratorium at this time, the Kremlin is seen as going over the head of Western governments in order to turn European sentiment against the missiles.

This approach has angered senior Bonn officials, who charge the Kremlin with undermining the seriousness of its offer to negotiate by such transparent attempts.

In a background discussion today, a senior Bonn diplomat described three phases to the Kremlin's strategy to foil the missile decision during the past 18 months. First, before the December 1979 decision, he said, the Soviets tried a "head-on" attack, warning the West that adoption of the missile deployment would preclude the eventual negotiations that were eagerly sought by the West Europeans.

After the NATO decision was made, the Soviets refused to discuss the matter for half a year. In July 1980, Brezhnev told West German Chancellor Helmut Schmidt, who was visiting in Moscow, that the Soviet Union, was ready to negotiate a wide range of weapons, including the missiles both sides had decided to deploy.

Brezhnev's latest offer to talk -- now linked to the moratorium proposal -- was said by the Bonn diplomat to represent a third phase that is a combination of the first two.

"The Soviets are trying both," the West German said, "saying they are ready to negotiate while attempting to undermine Western government positions through a direct appeal to public opinion."

To contain the mounting controversy in Europe about the missiles and protect the NATO decision, West European ministers in recent visits to Washington have urged the Reagan administration to set aside opposition to early arms controls talk and to resume negotiations.