Soviet Politburo member Viktor Grishin tonight sharply attacked the Reagan administration's policies and declared that the Soviet Union "is undertaking all necessary measures to strengthen its defense capability."

Speaking at the traditional Kremlin rally on the eve of celebrations of the 65th anniversary of the Bolshevik Revolution, Grishin assailed the "adventurist" policies of the Reagan administration, whose "aggressive" actions have created "the danger of a nuclear war."

"Our country," he said, "does not accept anyone's right to military superiority. If an attempt is made to gain it by deploying in Europe hundreds of new American nuclear missiles, or through some other means, the Soviet Union will adopt the necessary countermeasures."

The Soviet Union, he said, is strong enough to protect itself and support its allies, "from Cuba in the west to Vietnam in the east."

Although Grishin's speech followed the general policy line set by President Leonid Brezhnev last week, his tone was more aggressive than anything heard in recent years from a top Kremlin official.

He made no mention of possible improvement in Soviet-American relations and accused the Reagan administration of using proxies to destabilize Afghanistan, Angola, Ethiopia, Mozambique and Nicaragua. He also accused Washington of gross interference in the internal affairs of Poland.

He said that "it is especially obvious at present" that the United States wants to "bury detente," "revive the Cold War" and achieve military superiority over the Soviet Union.

"In the conditions of growing aggressiveness of imperialism, our party is displaying high vigilance and is undertaking all necessary measures to strengthen its defense capability," he said.

Diplomatic observers here noted that the principal emphasis of the speech was on the Soviet armed forces and their ability to deal with challenges posed by the "imperialists." This suggested that an emerging hard-line attitude here has gained the upper hand in Kremlin councils and that this is likely to be reflected in increased military expenditures in next year's budget.

Senior Western diplomats here believe that the armed forces are pressing for several new weapons systems, including a cruise missile and new missile-launching submarines.

Grishin, who is not a member of the inner leadership circle, repeated Brezhnev's recent conciliatory remarks about China and reaffirmed Moscow's position that "there is no type of weapons which our country would not be ready to restrict or ban on a reciprocal basis."

But the generally aggressive tone of his speech was interpreted by political observers here as an indication that the Kremlin does not expect any easing in Soviet-American tensions and that it is preparing the country for harder times.

Brezhnev and seven other Moscow-based Politburo members were in the audience at the Kremlin Palace of Congresses. Notably absent was Andrei Kirilenko, whose picture was dropped from a lineup of portraits of Soviet leaders to signal the end of his political career.

Also missing at the rally was Arvid Pelshe, at 83 the oldest member of the ruling body. His portrait remained in the lineup, however.