The Soviet Union enlisted the backing of 10 of its closest allies today for a new propaganda offensive against President Reagan's anticommunist "crusade" and particularly against the North Atlantic Treaty Organization's decision to deploy new U.S. nuclear missiles in Western Europe.

An official statement issued at the end of a two-day meeting of Central Committee secretaries from the seven Warsaw Pact countries and from Cuba, Mongolia, Laos and Vietnam warned that the proposed missile deployment had "decisive significance" not just for Europe but for "the whole of our planet."

The statement called for a broad effort to take "practical steps" to counteract Washington's anticommunist activities. It indirectly foreshadowed a new peace offensive by the Soviet Bloc.

"The participants in the meeting stressed the striving of the Communist and workers' parties of the socialist countries for the broadest possible dialogue and cooperation on problems of peace and disarmament with all the political and public forces, parties and organizations adhering to different ideologies, having different persuasions and views, but prepared to work for these ends," the statement said, according to the Soviet government news agency Tass.

It emphasized that deployment of new U.S. medium-range nuclear weapons in Western Europe, due to begin in December, would worsen the international situation.

"For this reason," the statement said, "it is exceptionally important to decisively oppose the unleashing of a new round of the nuclear arms race in Europe."

The meeting was the first of its kind since Yuri Andropov came to power here in November. Andropov met today with senior Communist officials who attended the conference, which was chaired by Politburo member Konstantin Chernenko.

Central Committee secretaries in charge of ideology and international questions are senior in rank to foreign ministers. The session this week was seen here as an effort to brief Moscow's allies on Soviet foreign policy and to coordinate support for the Soviet policy.

The Soviet Bloc countries also emphasized that Europe must respect "territorial-political realities" on the continent after World War II and referred to the present East-West divisions as "an integral and indispensable factor" of European peace and security. In Soviet diplomatic parlance this means the continued partition of Germany.

The statement welcomed the results of the summit of leaders of the Nonaligned Movement in New Delhi last week. The Soviet media gave prominence to the conference throughout the week, and commentaries indicated that the Soviets were clearly pleased by the nonaligned positions as well as by the fact that they escaped with only light criticism for the Soviet role in Afghanistan.

The conference also followed up on a declaration adopted at a Warsaw Pact summit in Prague two months ago by reaffirming an offer to sign a nonaggression pact with NATO and by renewing calls for an agreement to reduce military budgets on both sides.

Diplomatic observers here said Moscow has been disappointed with a lack of response in the West to those proposals. They suggested that the meeting of senior Central Committee secretaries may constitute an attempt by the Soviets to break out of their diplomatic isolation.