On an overcast October day in Tokyo in 1979, a broad-shouldered Russian and a gray-haired American boarded a Pan Am flight to Los Angeles while Japanese police stood guard. As they flew across the Pacific, the two new-found friends drank rounds of champagne in the first class section. What they celebrated received little notice in the western press, but was of great importance in Moscow and Washington.

The Soviet passenger was Stanislav A. Levchenko, a major in the Soviet secret police, the KGB, who had been working as a spy under the cover of a Soviet journalist in Tokyo. Hours before the flight he became one of the few known KGB operatives ever to defect to the United States. His American companion was an intelligence officer escorting him to this country.

Levchenko brought with him a wealth of knowledge about the KGB's activities in Japan. Testifying before a House committee last year, CIA deputy director John McMahon said the information he provided "was so damaging to the Soviet cause that it would be inconceivable that he might be under Soviet KGB control."

For the past 3 1/2 years, Levchenko, sentenced to death by a Soviet court in August 1981, has lived a secretive life in the United States, making only a few public appearances and frequently using an assumed name.

Now he is beginning to break his silence. In two lengthy interviews with The Washington Post, his first contacts with an American newspaper, Levchenko, 41, described how a model Soviet citizen with a promising career in the KGB first became a "hidden dissident" and then embraced a country he had been taught to hate.

He spoke in colloquial but accented English, and chain-smoked throughout the interviews. With a pear-shaped, mustachioed face, he revealed a disarming sense of humor and deftness at answering questions. But other times he resisted giving information about apparently innocuous details of his life here.

While charming and likable, Levchenko can also be intensely serious. He calls himself a "fighter," never failing to remind people that he is "straight," "honest," and "moral." An independent man with deep convictions and an obsession with detail and choice of words, Levchenko has avoided the press, partly out of fears that his "message to the free world" would be trivialized or misstated.

Since arriving in this country, Levchenko says he has tried to become "just another ordinary guy in the street." He says he spends a lot of time in northern California and regularly attends services in a Russian Orthodox church in the Napa Valley. He hopes to become a university professor.

But as part of a quixotic "declaration of war" against the Soviet government, he has surfaced under his real name to lecture to government, military, and academic audiences about the KGB and the Soviet Union.

Revelations about Levchenko's work in Japan in a new book on the KGB by Readers Digest editor John Barron have shaken the Japanese government, which says his information is "highly credible in its entirety."

Part of the motivation for this self-styled war is what he calls the "barbaric" persecution of his wife, Natalia, an architect, and his 18-year-old son, Alexander, whom he left behind in Tokyo. They returned to Moscow and have been refused permission to join him here. Levchenko said he learned through "private channels" that his son was expelled from school, the family car and bank accounts were seized and his wife was beaten up by "thugs" six months ago.

For the ex-KGB officer who spent nearly a decade in classic undercover work, some espionage habits die hard. He insisted that his first interivew with The Post be conducted in a motel that he would name only two hours in advance. In restaurants he still prefers to sit so he can see the entrance; eats his meals quickly, and just to be safe, sometimes "cleans" his tracks when returning from public appearances.

Although he says he is not afraid of being assassinated by Soviet agents, he considers himself "in a combat situation" and uses a false identity in private life because he doesn't want to "make their job easy."

But for the most part he appears to have adapted easily to American life. In California he has succumbed to popular western temptations, and owns a Walkman stereo set and an exercycle. He goes to Woody Allen movies and dresses casually, wearing an ID bracelet and two gold rings. In 1980 he took off in a Chevy Monte Carlo on a free-wheeling tour of the United States with a fellow defector, MIG pilot Viktor Belenko.

His life style now is hardly what was envisioned for the Soviet boy who was born into a military family in Moscow in 1941 and whose youth was filled with the orthodoxies of socialism.

Levchenko received an elite education reserved for the most promising students, graduating from Moscow's Institute of Oriental Languages. There he began to suspect that 20th century Leninism had distorted Marx's 19th century "idealistic" principles.

"I had fantasies at that time that probably sometime in 20, 30, or 50 years the time will come when something like democratic socialism can be established in the Soviet Union. It took me another 20 years to be absolutely sure it's impossible."

In his late 20s the religious "vacuum" he first felt in the university intensified, and secretly he began visiting churches. "My first interest was really cultural, but then I started to listen to what the priests were talking about and what the choir was singing about and I came to my own personal conclusions that Christianity is the best way and can give you lots of strength."

After working for the Soviet Peace Committee and the Afro-Asian Solidarity Committee, Levchenko began doing tasks for the KGB in 1968, spying on visiting guests, including two young Japanese diplomats in Moscow. In 1971 he began working for the KGB full time and was transferred to the First Chief Directorate, which does overseas espionage. He went to Japan in 1975 as a KGB intelligence officer, using the cover of a correspondent for the Soviet magazine Novoye Vremya (New Times).

During his four years in Tokyo, Levchenko says he quickly rose in the ranks of the KGB, perfecting the art of espionage. He told a House panel last July that besides acquiring intelligence on Japan, the KGB there sowed distrust between the Japanese and Americans in order to weaken U.S. influence. He was in charge of 10 Japanese agents, four of whom he said he recruited himself. They included journalists, an editor, members of parliament and a high-ranking official of the Japanese Socialist Party.

Levchenko said he was often able to get information by telling his Japanese recruits that he was collecting data for a confidential bulletin circulated only among high-ranking Politburo members. Flattered, the Japanese often cooperated, but the bulletin never existed.

According to Levchenko, his espionage coups in Japan included advance knowledge of the Chinese invasion of Vietnam in 1979; early warning of the disclosure that Japanese businessmen took bribes in the Lockheed scandal; information about the activities of certain U.S. forces on Okinawa, and U.S. plans to back a new Cambodian government under Prince Norodom Sihanouk.

But with his increasing success as a spy came growing disillusionment with the KGB, to the point that Levchenko felt it was "ruthless" and "free from any moral principles."

By the fall of 1979 Levchenko was due to return to Moscow. "I had come to a total resentment of the system. And what could I do? Go back and go ahead with my KGB career? Or start speaking up? They would put me in madhouse; just lock you in madhouse, right away; they will put you in jail. Because they cannot admit that KGB officer can hate the system," he said.

His decision to defect was not made hastily. "You've got to be sure that you resent the Soviet socialist system; you've got to be sure that you can't stand it anymore and you want to fight the system.

"The other thing is the fact that KGB is revengeful in very barbaric ways. Relatives will be hurt . . . The third thing, you've got to get some guts because potentially you are a dead man. So you've got to be basically a fighter, you've got to be honest to yourself and come to this decision within yourself."

Levchenko wanted to seek political asylum in the United States but did not know any Americans in Tokyo well enough to approach.

"I didn't want to go to the U.S. Embassy because, first of all, it is highly guarded, primarily by Japanese police," Levchenko said. "There are just a few Marines over there inside, but outside is Japanese police. Many of them probably knew my face already. The other thing was that I could end up talking to some young diplomat in the embassy who could be scared or confused, which would be a natural human reaction.

"So my logic said that probably the best way was to contact somebody military because it is in-built in military psychology that whatever happens, whatever you think about it, 'Go and report.' That is exactly what happened," he said, laughing.

He went to a hotel about 10 p.m. where American military personnel often socialized, found a party and asked to speak to a Navy captain whose face he recognized.

"It took me thirty seconds to explain to him who I am and what I want. He was silent for awhile," he said.

Feeling "very determined, very nervous, but not scared," Levchenko was taken to an empty room and told to wait with two military policemen. They offered him a beer. Five hours later, Levchenko was granted political asylum in the United States.

"When I came to this country it was a very great thing," Levchenko recalled. Mentally he had prepared himself for the possibility of a "horror interrogation-type thing" by U.S. intelligence officials. Levchenko said he took two polygraph tests and psychological tests when he first arrived, but that "nothing like a tough debriefing ever happened. Everyone welcomed me sincerely."

For a few weeks after his arrival Levchenko lived in an apartment in the Maryland suburbs with two U.S. security officials. Then he took a year-long job as a researcher for Reader's Digest, where author Barron works, and rented a studio apartment in Virginia.

During an extensive debriefing with U.S. intelligence personnel, sometimes in his apartment and sometimes in restaurants, Levchenko provided "extensive information on the Soviet system and on the activities of the KGB residency in Japan, identifying agents and assets he knew about," a CIA official told a House committee.

Some knowledgeable observers, however, are skeptical that Levchenko has been as valuable as the CIA testified. They said that U.S. intelligence officials and defectors sometimes have a tendency to exaggerate the value of information provided.

Initially, Levchenko said, he would not provide American intelligence agencies with a full-blown picture of KGB operations in Japan and the United States, in the hope that his family would be released. "I just had to wait for a while in the vain hope that the Soviet government would at least be reasonable about it," he said, although when he defected, the Soviet government had never allowed family members of a "traitor" to leave the country.

Levchenko's smoothly executed defection ran into its first snag in early 1980, when he was called to the State Department and asked by a high-ranking official to meet with representatives from the Soviet Embassy, as is customary with defectors.

"The conversation was quite unpleasant for me," said Levchenko, who did not want to meet the Soviets. "It was the only time I was under open pressure in the U.S. The first and only time."

Eventually he consented. Flanked by three Russian-speaking officials from the State Department's Soviet desk, Levchenko met with Alexander A. Bessmertnykh, now the second-ranking official at the Soviet Embassy, and two other Russians whom Levchenko said were KGB officers.

"They started to tell me that I can go back with them right away and they will put me on the nearest flight back to Moscow," Levchenko recalled. "And that my career over there will be beautiful and I can write my doctoral dissertation. All this ridiculous crap.

"Then they told me that they thought I was drugged and kidnaped in Japan. So I disappointed them by saying that neither of these things happened to me. [That] I was quite conscientious when I came here, I was looking forward to it actually. [And that] I wanted to ask for political asylum and that, thanks God, I don't have any plans to go back to the Soviet Union, period."

During the entire hour-long session, Levchenko says, his Soviet profession--spying--was never mentioned.

Realizing that his family was never going to be allowed to leave the Soviet Union, Levchenko said he demanded another meeting with Soviet officials.

"I declared war," Levchenko said. "I read them a written statement that I had prepared that I am declaring a war [against them] and I made it public.

"He [the head official] was listening to this thing and gradually, he was, you know, like a chameleon changing color. And finally he became tomato color, and then he became ripe tomato color. And then he said he didn't want to listen to any anti-Soviet lectures anymore and he became kind of nasty. Actually he went berserk and started yelling in a very shrieking voice that he didn't come to the State Department to listen to anti-Soviet lectures.

"While he was yelling I walked out of the room."

Eighteen months later, in August 1981, a Soviet court sentenced Levchenko to death.

Levchenko said that strains in his marriage prevented him from telling his wife about his defection. "At that time she was quite patriotic and she didn't know what was going on in my mind, because I couldn't share many things with her," he said. "The KGB must have known that I did have some problems in my marriage, not because of my wife, but because of myself. I was too busy all the time. I was probably not a great husband.

"But she is a very courageous person and, whether she was entirely happy about our family life or not, she just did not want to say derogatory things about me. And instead of human appreciation, she was punished badly for everything, which is absolutely barbaric. But that's the way things are basically in the Soviet Union."

Whatever suspicions have been aroused about the value of his information, Levchenko has a seriousness of purpose and an energy that continues to propel him into the future. He said he seldom sleeps more than five hours a night, and subscribes to 15 newspapers, which he tries to read daily.

In addition to teaching, he plans to write a book. He said he is earning enough to live comfortably, although he has no savings.

And he contemplates the fate of the people he left behind, a nation of "great" Russian people who, he said, are powerless victims of a vicious system.

"It's just a tragic chain of events that the system came into existence," Levchenko said. "It's impossible for me to imagine doing anything against my people. It is not the fault of the Russian people at all. It is a great people. By willingness to fight the system, we are not talking about fighting the people. It is very important to fight the system--for the people."

Levchenko's pronouncements have not gone unnoticed by Soviet officialdom here.

An official in the Soviet Embassy's press office, asked last week for information about Levchenko, promptly replied: "So you are doing another spy story and you want assistance from the Soviet Embassy? I don't think the embassy will assist in doing an anti-Soviet story."

Asked why an article on Levchenko necessarily would be anti-Soviet, the official laughed and said: "I don't think a story about Mr. Levchenko would be pro-Soviet, don't you agree?"