Andrei Dmitriyevich Sakharov made his first great reputation by inventing a hydrogen bomb, and his second by inventing a role for himself as a one-man defense force for the protection of human rights and human dignity in the Soviet Union.

"Force" is surely the right word, although no personal encounter with Sakharov would hint at its aptness. In person he is the mildest of absentminded professors, a gentle man who likes to drink his tea out of the saucer rather than the cup.

But in action he was a genuine force to be dealt with -- a dedicated, sometimes fanatic advocate for Soviet citizens who wanted to speak their minds but found that the regime's politicial police would not permit it. Sakharov's function, as he has seen it, was to serve as a witness for these Soviet citizens, and to do all he could to make known their plight.

The best evidence of his effectiveness, probably, was his arrest in Moscow yesterday by agents of that same political police force. The police could only act with authorization from the ruling Politburo itself, for that was the level on which the Sakharov problem aggravated the Soviet leadership. For a dozen years, he has eaten away at the official mythology about Soviet society like acid in a soft container.

But until yesterday, when the authorities he so annoyed sent him off to a silent internal exile in the grimy industrial city of Gorki, where no foreigners are permitted, Sakharov enjoyed special protection. The leadership did not know what to do with him.

He was, after all, a legitimate national hero, winner of the Stalin Prize, the Lenin Prize, and three times named a Hero of Socialist Labor. (He was stripped of all these yesterday. No living Soviet citizen outside the mutually self-congratulatory Politburo had received such honors.

In his tape-recorded oral "memoirs," Nikita Khrushchev said of Sakharov: "I knew him and was profoundly impressed by him. Everyone was. He was, as they say, a crystal of morality among our scientist." Not only that, he gave the Soviet Union the hydrogen bomb at a moment when Stalin was desperate to have it. He did not win those state prizes for being "a crystal of morality."

Sakharov became known in the West for his 1968 essay, "Progress, Coexistence and Intellectual Freedom," a plea for liberal, humane principles, and the eventual convergence of communism and capitalism in a system of universal democracy. But many years before he wrote this essay -- which he knew was a dangerous act, likely to transform his life as a Soviet scientist -- Sakharov had begun to agitate inside the sytem, particularly for control of nuclear weapons.

Though always extremely modest -- for example, Khrushchev credits him as the father of the hydrogen bomb, but Sakharov always shares the credit -- he acknowledges believing that he had some influence in 1962 on the test-ban treaty.

Soviet officials were hung up on the problem of the undetectability of underground nuclear tests. Sakharov proposed ignoring them, and drafting a treaty controlling only atmospheric testing. This was the idea incorporated in the 1963 treaty.

In 1968, after publication in the West of his essay on coexistence, Sakharov was removed from his secret positions, and his career as a research scientist effectively ended. But his career as a political activist had just begun.

Characteristically, he driftred into it slowly. As late as 1971 it was unusual for Sakharov to meet with a Western correspondent in Moscow, a form of behavior that was still unthinkable to most members of his generation. But his role was transformed during the early 1970s, and by 1973 he was holding regular press conferences in the small Moscow flat he shared with is second wife and her mother.

Sakharov's contest with the Committee for State Security, the KGB, was an exciting spectacle -- one man against a gigantic police force, one spirit against a system. And for a time, Sakharov seemed to be winning.

Inside the Soviet Union, millions of citizens learned about Sakhrov's views and activities. (Information about them was broadcast into the country by Western radio stations.) He won the Nobel Peace Prize. He helped countless Soviet citizens in distress.

But in the longer run it was an unequal contest. The KGB had too many resources. It put enormous pressure on Sakharov and his wife, and they both aged quickly. Sakharov lost his ebllient optimism. Still, the Politburo was afraid to arrest him, fearing the repercussions among Soviet scientists and in the West, where scientists of many lands have stood up for Sakharov.

But now, after a Soviet invasion of Afghanistan and the evident collapse of Soviet-American detente, the inhibitions have apparently disappeared., and Andrei Sakharov has been silenced, at least for a time.