The Soviet Union announced tonight that it will not take part in the 1984 Summer Olympic Games in Los Angeles because the Reagan administration "does not intend to ensure the security" of Soviet athletes.

A statement by the Soviet National Olympic Committee accused the Reagan administration of being in "direct connivance" with various extremist organizations seeking to create "unbearable conditions" for Soviet participants.

It said that "hostile anti-Soviet propaganda" and threats against Soviet participants were part of the administration's design to use the Olympic Games "for its political aims."

[In Washington, a White House spokesman denounced the Soviet decision as a "blatant political act for which there is no justification."]

Tonight's Soviet announcement, distributed by the government news agency Tass, capped a campaign of public complaints by Soviet officials and in the Soviet press about the activities of right-wing groups in Los Angeles that intend to urge Soviet athletes to defect.

In 1980, the United States led 54 other nations in a boycott of the Summer Olympics in Moscow in protest over the Soviet invasion of Afghanistan. Soviet spokesmen have stated that they had no intention of boycotting the Los Angeles Games in retaliation for the U.S. action.

The Soviet decision to boycott the Los Angeles Games was undoubtedly taken at the highest levels of Kremlin leadership and appeared to be aimed directly at President Reagan and his reelection campaign. The absence of Soviet athletes at the Summer Olympics seems designed to demonstrate to the American public the continued deterioration in Soviet-American relations.

"The National Olympic Committee of the Soviet Union is compelled to declare that participation of Soviet sportsmen in the games of the 23d Olympiad in Los Angeles is impossible," the Soviet statement said.

The United States had "gone the last mile" to accommodate the Soviets at Los Angeles, said White House spokesman Larry Speakes. Speakes said the Soviet statement did not appear to leave any room for a change of heart.

[State Department spokesman John Hughes said, "The conscience of the United States is clear. We have nothing to apologize for."]

The Soviet decision opens a distinct possibility that other Soviet Bloc countries may boycott the Los Angeles Games.

The Polish press agency, in a commentary this evening, said Poland's participation in the Summer Games was "under a question mark," Washington Post correspondent Bradley Graham reported from Warsaw. The article accused U.S. organizers of the Games and other American groups of attempting to use the event for political ends.

[Czech sources said they assumed Prague would stand by Moscow and withdraw. A spokesman for the Romanian Embassy in Vienna said his country would take part, but there was no immediate confirmation from the government in Bucharest. A Hungarian Olympic Committee spokesman in Budapest said Hungary would announce its decision in 48 hours, United Press International reported.]

A month ago, Moscow called for an emergency session of the International Olympic Committee to discuss alleged violations by the United States of the Olympic charter and to demand "effective measures to guarantee proper security to the participants in and visitors to the games."

That call was preceded by a meeting in Moscow of Central Committee representatives of all Soviet Bloc countries to discuss the Olympic Games. The meeting was attended by representatives of the Soviet Union, East Germany, Czechoslovakia, Hungary, Romania, Bulgaria, Poland, Cuba, Vietnam, Laos, Mongolia and North Korea.

Knowledgeable sources said at the time that if the Reagan administration failed to ensure security for the athletes and "normal conditions" in Los Angeles, most Soviet Bloc countries would boycott the Games.

Western and American diplomats have repeatedly insisted that the Russians were bluffing and that their repeated warnings could be discounted.

Today's statement makes clear that they were not bluffing and appears to represent a final Soviet position unless there is a marked and dramatic change in the U.S. position.

At an April 24 meeting between U.S. and Soviet Olympic officials in Lausanne, Switzerland, the Soviet delegation apparently insisted on "state-level" guarantees for the protection of their athletes against harassment, diplomatic sources in Washington said. The implication was that the Soviets would insist on a public commitment by the Reagan administration to keep the Games free of political overtones.

Judging by articles in the Soviet press, Moscow's main objection focused on alleged U.S. plans to stimulate hatred against the Soviet Union and purported programs to induce defections of Soviet and other Eastern Bloc athletes.

Moscow's decision to stay away from the Games, if followed by similar moves by other Soviet allies, especially East Germany, would seriously mar the Los Angeles Olympics, both competitively and financially.

The Soviet Union and East Germany traditionally are top medal winners at the Olympics, accounting for more than one-half of all medals. Their absence from Los Angeles would be a far heavier blow to the Games than was the partial boycott of the Moscow games by western nations.

Tonight's statement said that a meeting of the International Olympic Committee two weeks ago had found Soviet charges against the United States to be "just and substantiated."

"But disregarding the opinion of the International Olympic Committee, the United States authorities continue rudely to interfere in affairs belonging exclusively to the competence of the Los Angeles Olympic Organizing Committee.

At the April 24 meeting in Lausanne, the Americans agreed not to require the Soviets to submit a list of their Olympic delegation to U.S. Embassy officials, one of the Soviets' major complaints. After the meeting Peter Ueberroth, president of the Los Angeles Olympic Organizing Committee, said major progress had been made to resolve the Soviets' complaints and that he was convinced the Russians wanted to participate in the Games.

["The United States governemnt and the Los Angeles Olympic Organizing Committee have done everything possible to facilitate the participation of all nations in the 1984 Olympics," said White House spokesman Speakes. "I think the spirit of the Olympics will go on as it has for 22 previous other Olympiads."]

The Tass statement charged that "it is known that from the very first days of preparations for the present Olympics, the American administration has set course at using the games for its political aims. Chauvinistinc sentiments and an anti-Soviet hysteria are being whipped up in the country.

"Extremist organizations and groupings of all sorts, openly aiming to create unbearable conditions for the stay of the Soviet delegation and performance by Soviet athletes have sharply stepped up their activity in direct connivance of the American authorities."

It said that "hostile political demonstrations" were being prepared, "undisguised threats are made against the Soviet Olympic Committee, Soviet athletes and officials," and that leaders of "anti-Soviet and anti-socialist organizations" were being openly received by Reagan administration officials.

The statement dismissed U.S. assurances that the rules of the Olympic charter would be observed by saying that "the practical deeds by the American side show that it does not indend to ensure the security of all athletes, respect their rights and human dignity, and create normal conditions for holding the games."

All this, the statement said, shows the "gross flouting of the ideals and traditions of the Olympic movement" and "are aimed directly at undermining it."

The decision not to participate at Los Angeles was taken by a unanimous vote, Tass said. "To act differently would be tantamount to approving of the anti-Olympic actions of the U.S. authorities and organizers of the games."

In adopting this decision, the statement said, "we have not the slightest wish to cast aspersion on the American public, to cloud the good feelings linking sportsmen of our countries."

The Soviet Olympic Committee pledged to support the Olympic movement in the future and to "struggle for the preservation of its purity and unity."

Tonight's statement was preceded by letters from readers published in the press urging the Soviet authorities not to send the Soviet Olympic team to Los Angeles.

A few weeks ago, following the first Soviet statement that opened the possibility of a boycott of the Los Angeles game, there was an impression here that the Russians wanted to go to Los Angeles provided they get adequate U.S. "assurances" on the question of security for Soviet and East European participants.

Moscow had gone ahead with arrangments for the games, paying its television coverage fees and planning sea and air transport to Los Angeles.

The chairman of the Soviet committee, Marat Gramov, who holds the rank of Cabinet minister, said on April 16 that Moscow would not boycott the games. But he also asserted that the Soviet team might not participate in Los Angeles if the United States maintained its "discriminatory" measures against the East Bloc participants.

A spokesman for the Lausanne-based International Olympic Committee said the organization had been unaware of the Soviet decision. "All we know is what Tass has announced. There has been no official word from the Soviet Olympic Committee," he said.

The International Amateur Athletic Federation, the largest federation in the Olympic movement, also said it was not informed by the Soviet Union of its boycott decision.

In Stuttgart, a spokesman for West German Chancellor Helmut Kohl said the government hoped the Kremlin would change its mind. "I am absolutely devastated by this news," said Nelson Paillou, president of the French National Olympic Committee, UPI reported.