President Yuri Andropov today sought to mobilize West German opposition to the scheduled deployment of American medium-range nuclear missiles in his first public statement since the downing of a South Korean airliner by the Soviet Union.

The Soviet leader was quoted by the official Tass news agency as saying that everything must be done to prevent West German territory from becoming the possible source of a new war. His statement came in the form of a letter replying to a reported appeal by a group of opposition Social Democrat deputies in the West German Bundestag, the lower house of parliament, for a halt to the arms race.

Andropov's letter as reported by Tass made no mention of the downing of the South Korean Boeing 747 and was couched in moderate language by Kremlin standards. It played on the report that, according to a recent public opinion poll, 72 percent of West Germans are opposed to the stationing of American cruise and Pershing II missiles.

Depicting the Soviet Union and West Germany as the parents of detente, Andropov said that the "people of our countries . . . cannot look indifferently on how attempts are being made in the name of interests alien to Europe" to destroy all positive achievements in East-West relations.

Publication of Andropov's letter coincided with a meeting here of senior Communist Party officials from all over the Soviet Bloc to discuss a common strategy as the December deadline for deployment of the U.S. missiles in Europe approaches. Participants in the meeting included party secretaries for ideology and international affairs.

In an apparent reference to the plane incident, a communique released by Tass after the meeting said the United States was attempting to use "provocative actions" to ensure the implementation of its missile plans.

Western diplomats here said the purpose of the meeting appeared to be to assign specific tasks to each Soviet Bloc country as the propaganda war heats up over the next few months.

The timing and subject matter of Andropov's letter were significant, Western diplomats said today, even though there was nothing substantially new in what he had to say.

The letter is Andropov's first public statement since Aug. 27 when he offered to "liquidate" some Soviet SS20s as part of a package agreement with the United States on medium-range nuclear weapons in Europe. He repeated the proposal today.

The letter, the diplomats said, is another indication that the Kremlin is trying to shift world attention back to the stalled nuclear arms negotiations in Geneva following the international furor over the airliner.

Andropov, who is believed to have resumed his holiday in the Caucasus after flying back to Moscow because of the airliner crisis, has kept a low profile over the last few weeks. His silence has been interpreted here as a tactical move designed to distance himself politically from the decision to shoot down the plane in order to lend greater credibility to his arms control initiatives.

The letter was interpreted here as a tentative attempt to restore the Soviet Union's good image among western peace groups after the propaganda setback of the plane incident. The Kremlin regards West Germany, which is scheduled to receive 108 Pershing II and 96 cruise missiles, as the key to the missile controversy in Europe.

Tass did not identify the Social Democrat deputies who wrote to Andropov and the West German Embassy here was also unable to give their names. They were quoted as being "alarmed" by the possibility of a dramatic escalation in the arms race if the Geneva talks failed.