Soviet President Leonid Brezhnev declared today that his country will deal "a crushing retaliatory blow" to any aggressor and that "our might and vigilance will cool, I believe, the hot heads of some imperialist politicians."

He made the remarks at a Kremlin reception shortly after his defense minister, Marshal Dmitri Ustinov, opened the dazzling annual show of military power in Red Square by directly accusing the Reagan administration of having "unleashed a political, ideological and economic offensive against socialism" and having raised its "military preparation to an unprecedented level."

While Brezhnev reaffirmed his commitment to detente and arms control, the tone of his speech was unusually belligerent and was taken by diplomats here as another indication of a new tough anti-American line in the Kremlin.

Speaking to Soviet dignitaries and foreign diplomats attending a reception in honor of the 65th anniversary of the Bolshevik Revolution, Brezhnev said that "it is not the tradition of our party and our people to retreat before difficulties.

"We shall do our utmost to see to it that the proponents of military adventurism should never catch unawares the land of the Soviets and that the potential aggressor should know that a crushing retaliatory strike will inevitably be in store for him."

U.S. Ambassador Arthur Hartman was among foreign diplomats attending the Kremlin reception although he and most other NATO envoys boycotted the military parade in Red Square to protest the Soviet invasion and occupation of Afghanistan.

The ambassadors of Turkey, Greece, Denmark and Norway did attend the military parade this year. The Chinese ambassador attended the parade for the first time in many years, illustrating the thaw in relations between Moscow and Peking.

Brezhnev stood for a full two hours on top of the Lenin mausoleum despite freezing temperatures and a biting northern wind to preside over a spectacle of his country's power staged on a grand scale.

From the lineup of his fur-hatted Politburo colleagues atop the mausoleum it was clear that Konstantin Chernenko and Yuri Andropov are currently the key figures in the leadership after Brezhnev.

Brezhnev, 75, watched units marching 20 abreast and followed by tanks, armored personnel carriers and vehicles carrying rockets. He looked fit and drank occasionally from a paper cup.

Western military specialists said no new hardware was shown today except a modified armored personnel carrier with a new turret that was first sighted earlier this year in Afghanistan. Also displayed for the first time in a parade was an SA8B truck-mounted surface-to-air missile system. Western observers said the SA8B is an improved version of the missiles used by Syria that failed to perform well against Israeli planes in Lebanon.

A military band of more than a thousand musicians played martial music for the military part of the show. It was followed by tens of thousands of Muscovites filling the vast square carrying paper flowers, balloons and red banners.

Ustinov, the only speaker at the parade, talked about the need to strengthen Soviet military defenses. But while in previous years he avoided specific mention of the United States and instead talked of the threatening actions of "imperialists," Ustinov this time singled out the United States as having "raised their level of military preparations to an unprecedented level."

"They have unleashed a political, ideological and economic offensive against socialism," he said.

He charged that the international situation continues to worsen because of Washington's policies promoting "the arms race, impudently interfering in the affairs of other countries and facilitating outbreaks of military conflicts in various parts of the world."