The Soviet Union plans to hold massive antiwar demonstrations here on Saturday in what appears to be the start of a new propaganda campaign to blame the United States for heightened international tensions.

The demonstrations were announced today by the official Soviet news agency Tass, following a statement by Soviet leader Yuri Andropov yesterday lambasting the Reagan administration for its foreign policies. The rallies are being organized by the Soviet peace committee at 15 sites around the city.

Soviet media have stepped up their attacks on the United States over Lebanon, arms control and the downing of the South Korean airliner. Television news carried interviews with workers and intellectuals, and the leaders of such countries as Afghanistan and Outer Mongolia have been called upon to give statements supporting specific themes in Andropov's speech.

The government newspaper Izvestia said today that the U.S. administration had "sacrificed" the 269 passengers on board the South Korean jumbo jet in an anticommunist crusade. It said that Washington was seeking to torpedo arms control negotiations in Europe and find new pretexts for armed intervention in the Middle East and Central America.

Western diplomats here said that holding the demonstrations is an unusual way of mobilizing public opinion behind Soviet leaders on a sensitive issue of foreign policy. Peace rallies have taken place in Moscow and other Soviet cities in the past, but never on the scale planned for Saturday's demonstrations.

In an attempt to guarantee that the demonstrations receive wide publicity in the West, pictures of the event have been promised to foreign television companies.

Meanwhile, it remains unclear whether Andropov has returned to Moscow following a holiday in the Caucasus. Tass reported today that he met with the president of South Yemen on Wednesday, but no pictures of the meeting were carried by television news and its site was not given.

Yesterday, a Soviet official said in a private conversation that he believed Andropov is still away from Moscow.

The last time that the Kremlin leader has been seen in public was in mid-August, when he met a group of U.S. congressmen. Since then, Andropov has issued several important statements and signed messages, but no news photographs of him have appeared.

A senior Western diplomat said that Andropov's whereabouts are a mystery, but attached no particular significance to this. The diplomat noted that, unlike his predecessor Leonid Brezhnev, Andropov shies from personal publicity.