Soviet Defense Minister Dmitri Ustinov today blamed the West for seeking to use "the events in Poland" to undermine the entire Soviet Bloc and accused the Reagan administration of encouraging "international gangsterism."

"All these actions create a threat to the security of socialist" countries, Ustinov said in a nationally televised speech. But, he added, the Soviet armed forces along with those of their allies stand ready to defend "the gains of socialism and the interests of socialist community."

Ustinov, dressed in a marshal's uniform, spoke at an unspecified location in Byelorussia at the end of major Soviet military maneuvers conducted for the past seven days around the borders of Poland. He was flanked by all major Soviet military commanders and the defense ministers of the Warsaw Pact countries and Cuba, Vietnam and Mongolia.

Polish Premier Wojciech Jaruzelski, who also is defense minister, was on the podium overlooking a vast area where tens of thousands of Soviet troops and thousands of tanks and other heavy armor were gathered.

While it is customary for Soviet defense ministers to deliver hard-hitting speeches, Ustinov's charge that the West was trying to interfere in Poland was interpreted by political observers here as a new warning that Moscow would not permit a radical change of the Polish political system.

Ustinov's speech and the maneuvers involving 100,000 troops in Byelorussia and the Baltic republics and about 80 naval vessels in the Baltic Sea were also seen as an effort to ensure that Moscow's restraint during the past year concerning the challenge to Communist Party rule in Poland not be interpreted as a sign of weakness.

Meanwhile, Soviet authorities have stepped up an internal propaganda campaign against Poland's independent trade union Solidarity with the publication of open letters to Polish workers asserting that the defense of socialism was the "joint sacred duty" of the Polish and Soviet peoples.

The campaign got under way yesterday with mass meetings in major factories and enterprises throughout the Soviet Union and the publication of a letter to the Polish workers endorsed by 70,000 workers of the Zil automobile plant in Moscow.

Today, the news agency Tass published several other letters to Polish workers, including one approved by 100,000 workers of the Kama River truck plant.

The internal propaganda campaign was in part designed, according to political observers here, to put pressure on the Polish government and ruling Communist Party to crack down on Solidarity and prevent the union from holding the second stage of its congress. The preliminary stage of the union's first national congress ended Thursday.

It was also seen by Western diplomats as possibly preparing the Soviet people for an intervention should the Polish government fail to combat "counterrevolution." The letters and press commentaries do not contain statements couched in the oblique terms earlier favored by Moscow when criticizing the course of events in Poland but instead use harsh language and outright condemnations.

One letter today, for example, referred to "the malicious leadership of Solidarity." Another refers to them as "the accomplices of imperialism." All express the hope that the Polish loyalists would move into action "to parry" Solidarity activities and "prevent them from eliminating the revolutionary gains" in Poland.

In his speech today, Ustinov came close to issuing a direct warning to the Poles when he declared that the war games have demonstrated that the Soviet forces are ready "to reliably defend our socialist motherland, our friends and allies."

He continued: "Imperialist reaction is striving to undermine in every way the foundations of the social system in countries of the socialist community. This is shown specifically by the West's constant attempts to interfere in the events in Poland."

He then cited President Reagan's decision to produce a neutron weapon, the proposed deployment of intermediate-range U.S. nuclear weapons in Europe and what he described as China's collusion with the United States in increasing international tensions.