Arkady Shevchenko, the highly placed and high-living Soviet diplomat who defected to the United States in 1978, spoke out for the first time publicly yesterday and called on the American people to "unite" behind an Olympic boycott to help "awaken the population of the Soviet Union to the evils of their regime and its aggressive policies."

"The magnitude of the threat to world peace by the Soviet Union," he said, "is such that if it is not punished in a specific, concrete and forceful way for its invasion of Afghanistan, the world will find itself in the not too distant future under the domination of Moscow."

The 49-year-old diplomat, a former undersecretary at the United Nations in New York and the highest-ranking Soviet official known to have defected, appeared yesterday at a House Intelligence subcommittee hearing on Soviet internal policies and warned that the Kremlin was heading into a frightening era of repression at home and domination abroad.

Shevchenko has stayed out of public view for much of the time since he defected, during which he has been under the protection and financial care of the CIA. A CIA spokesman said yesterday that Shevchenko is "pretty much of a private citizen" now, though still an active consultant for the agency.

He was invited to appear in public by the committee two weeks ago, after the Soviet invasion of Afghanistan, and yesterday he delivered an unrelenting attack on the values and objectives of Soviet leadership in a prepared statement and during a subsequent question-and-answer session with committee members.

Shevchenko said the role and power of the KGB, the Soviet secret police, was again reaching "the darkest days of Stalin," the former Soviet ruler, "when millions of innocent people became the victims of terrible crime."

Typical of this growing power, he said, was the bold decision to send Nobel Peace Prize-winner Andrei Sakharov into internal exile without the semblance of a trial.

"They can do everything -- follow you, bug you, send you away, open your mail, detain and arrest you, send you to a metal institution -- all without a warrant."

The Soviet leadership he said, "is always ready to go to extremes to suppress dissent and criticism of the system and now the situation is such that they have decided to crush the whole dissident movement."

Rep. Norman Y. Mineta (D-Calif.) asked why, in the midst of so much global criticism, the Soviets would move openly against Sakharov, a nuclear physicist. Shevchenko explained that it was precisely because of the "anti-Soviet hysteria in the world now" that the Kremlin decided the time was "appropriate" to exile Sakharov, who they have always wanted to silence.

For one thing, his voice is heard not only in the West but also inside Russia as well and the authorities would be uncomfortable with him around.

Perhaps more importantly, the Soviets now reason, he said, that they are no longer going to get anything out of the United States, so they can exile Skharov. They would not do it, he said, in a period of detente where they wanted something like a strategic arms agreement.

Questioned about the Olympics, the gray-haired former diplomat said Moscow "is the last place in the world where politics is separated from sports."

Shevchenko said the objective of the Moscow Olympics, in Soviet eyes, was to have "a huge political show" meant to portray the "correctness of their policies."

Many Soviet people, he said, probably believe the official "lies" that the action in Aghanistan was in resistance to U.S.-backed agitation. But lack of U.S. participation in the Games, he said, would be a staggering jolt to the leadership and probably help the population realize there was more to the situation than the party line.

Shevchenko's defection unfolded suddenly and dramatically when he resigned his $76,000-a-year job in New York on April 6, 1978, and asked for asylum in the Unted States soon after receiving orders from Moscow to come home. He lived well in New York and apparently enjoyed it.

In october 1978, the spotlight shown on him again, only this time it was a Washington woman, Judy Chavez, who told reporters that Shevchenko had spent some $40,000 on her for companionship as an escort during a Caribbean tour. The CIA and the White House denied it was government money. Shevchenko says it came from his U.S. severence pay.

Shevchenko's wife had been whisked back to Moscow at the time of his defection and she died there later in 1978 under what he described yesterday as mysterious circumstances.

The Russian, once one of the youngest members of the Soviet elite, arrived in the small hearing room yesterday with his second wife, Elaine, an American, and his lawyer.

He said he wanted to set straight press speculation about his motives for defecting and to reject the Soviet claim that he had been "captured by the CIA."

Shevchenko said he left because, after living in the United States, he had had the opportunity to compare. "The Soviet regime," he says he came to understand, "represents the antithesis of the ideals which it professes at home and abroad."

He said "an ominous war machine of immense proportions is now in existence" and that the longer he served and the higher he rose in the diplomatic serivce " the more I was convinced it was not a policy of peace but a policy of aggression, expansion and enslavement of other people."

Finally, he said, explaining his crucial decision, "my intellectual journey began to create in me a revulsion akin to physical sickness. The privileges of my high position brought neither ease nor contentment, for there is a kind of pain which grows in the mind as one begins to become aware that he is part of a cruel and dangerous society.

"More and more I thought about my fellow countrymen who had experieced this awakening -- that stream of the finest representatives of Soviet intelligentsia, writers and artists who have turned their backs on the Soviet Union and sought new lives in the West. And, finally, I decided to act," he said.