Andrei Dmitriyevich Sakharov, 68, who died in Moscow Thursday night after a heart attack, was both the father of the Soviet Union's hydrogen bomb and a human rights activist of global renown. His fearless stance against Soviet totalitarian repression helped spark the political reforms now sweeping Eastern Europe and his own homeland. His work as a nuclear physicist earned him extraordinary honors from the Soviet regime and the awe of the international scientific community. But his greater mark in history will be as a spokesman and activist for human rights, who despite reprisals from his own government won a Nobel Peace Prize for steadfastly advocating political freedom and democracy as the only rational way to insure against nuclear holocaust. At his death, Dr. Sakharov was perhaps the most widely regarded citizen of his own country, revered by millions of Soviets as a man of peace, vision and compassion. His example of courageous activism on behalf of pluralistic democracy had served as a model to reformers in many countries ruled by authoritarian leaders. Dr. Sakharov was elected a member of the Congress of People's Deputies this year, a remarkable event for a man who had spent most of the 1980s under house arrest in the provincial city of Gorky, 250 miles east of Moscow. As a member of the body, Dr. Sakharov spoke out frequently for democratic reform, engendering the ire of Mikhail S. Gorbachev, who had freed Dr. Sakharov from exile in December 1986, in a dramatic gesture that won Gorbachev worldwide praise. The unusual moral force of Dr. Sakharov's activism derived from the fact that having played an indispensable role in putting nuclear weapons into the hands of his government, he came to believe that the Soviet regime's refusal to allow human liberties to flourish in the country made the Kremlin a threat to world peace. From 1968, when he emerged from the secrecy-shrouded Soviet nuclear weapons program to speak out publicly against political repression, Dr. Sakharov's views commanded the attention -- and agreement -- of millions worldwide. A tall, stoop-shouldered man with a gentle, withdrawn demeanor, Dr. Sakharov and his bluntly stated demands for free speech, free assembly and other fundamental human rights were well known to millions in the Soviet Union and throughout the world. His global stature came about despite the fact that he was forbidden throughout most of his life to travel abroad. But his ideas on the moral imperatives of freedom and society based on the rule of law were disseminated widely by the media of many nations, the kind of exercise of free speech Dr. Sakharov advocated. In the early years of his activism, Dr. Sakharov's appeals on behalf of imprisoned political and religious activists and essays on political freedom were broadcast into the Soviet Union by Western radio stations, such as the Voice of America and the BBC World Service. They were heard by millions despite years of Soviet electronic jamming, which Gorbachev ended some years ago. The spread of Dr. Sakharov's views within the Soviet Union and the East bloc was aided by the underground self-publication, or samizdat movement. He thus became the most influential and respected voice of political dissent in the Soviet Union in the 1970s, a decade remarkable for the flowering and subsequent repression of disparate anti-regime activism. From the first moments of his emergence as a political activist two decades ago, until Gorbachev's intercession to end the exile, Dr. Sakharov was treated as an official pariah and vilified as a state traitor. The attempts to muzzle Dr. Sakharov culminated in his arrest early in 1980 after he publicly denounced the December 1979 Soviet invasion of Afghanistan and called on the United States and other countries to boycott the 1980 Summer Olympic Games, to be held in Moscow. Dr. Sakharov was seized by police on a snowy Moscow street, and in a kangaroo-court proceeding, stripped of his many state awards and honors, and dispatched to internal exile. He and his wife, Yelena Bonner, were housed in a small apartment closely guarded by the KGB secret police. Sakharov's Soviet friends and scientific colleagues were forbidden to see him in Gorky, a city from which all foreigners were barred. Mail to him was intercepted and telephone calls were banned. This painful exile, whose debilitating mental and physical effects were intensified by two hunger strikes he and his wife conducted to win travel rights for their family members, was ended by Gorbachev in dramatic fashion. In December 1986, telephone service to the Sakharov apartment was suddenly activated, and the Soviet leader telephoned the exiled dissident. Soon, Sakharov was on his way back to Moscow; his arrival was greeted by throngs of activists and ordinary citizens. But in the three years since, Gorbachev's initial attitude of wary regard for Dr. Sakharov faded, replaced by irritation at the physicist's unremitting criticism of the slow pace of political and economic reform. In addition, Dr. Sakharov engendered Gorbachev's ire by joining the leadership of a group of newly elected Soviet members of parliament who are seeking to end the Communist Party's constitutionally guaranteed monopoly on political power. Dr. Sakharov's worldwide reputation rests on a series of essays of political thought, plus dozens of statements and appeals defending human rights activists in the U.S.S.R.and other countries jailed in labor camps or psychiatric hospitals for their views. His first work, a series of 12 philosophically linked essays that broadly discuss the need for political freedom, appeared in 1968 in samizdat. Entitled "Progress, Coexistence and Intellectual Freedom," the essays were smuggled out of the country and published later that year in the West, where they caused a sensation. Here was an inquisitive, powerful intellect, nurtured exclusively within the repressive, anti-democratic Soviet system, striking telling blows for freedoms that Lenin and his successors up to that point had never allowed. The voice was fresh, utopian, hopeful. The global ills he spoke of nearly two decades ago remain today: threat of nuclear war, hunger, racism, overpopulation, environmental pollution, totalitarianism. He called for an extra-national dedication by mankind to the cause of peace, declaring in part: "All people have the right to decide their own fate with a free expression of will . . . ." Optimistically -- and incorrectly -- he foresaw that by 1980 his country's one-party Communist regime would have voluntarily modified itself into a pluralistic political system. He thought this development would be paralleled by the emergence of leftist reform within the capitalist camp that would pursue rapprochement with the socialist bloc. In a rueful retrospective years later, Dr. Sakharov concluded this first work had been naive and intellectually shallow. This harsh self-criticism overlooked an essential truth: the essays established Sakharov as a figure of global dimensions. Perhaps nowhere were his yearnings for world justice and freedom more powerfully stated than in his 1975 Nobel lecture. Yelena Bonner, who had temporary permission to travel abroad for medical treatment of glaucoma, an eye disease, delivered the address to the Norwegian Parliament in Oslo that December on behalf of her husband. He declared his conviction that "international trust, mutual understanding, disarmament and international security are inconceivable without an open society with freedom of information, freedom of conscience, the right to publish and the right to travel and choose the country in which one wishes to live." He rejected Marxist dialectics, and asserted his belief in "the original and decisive significance of civic and political rights in shaping the destiny of mankind." Democratization of the Soviet system was essential to world peace, he said, and asked that all mankind take up the struggle for human dignity: "We should not minimize our sacred endeavors in this world, where, like faint glimmers in the dark, we have emerged for a moment from the nothingness of dark unconsciousness into a material existence. We must make good the demands of reason and create a life worthy of ourselves and of the goals we only dimly perceive." The man who saw life in the nuclear age in such protean terms was born in Moscow on May 21, 1921, into a family of Russian intellectuals. His father, Dmitri, was a well-known physics teacher, an excellent classical pianist and author of popular physics texts. Andrei grew up a serene, dreamy youth, protected from the tumultuous early years of Soviet rule by his large, close-knit and loving family. But when Joseph Stalin launched the purges of the 1930s, Sakharov's family suffered along with millions of other Soviets: six uncles and aunts were arrested on false political charges and four of them died in the Gulag slave-labor camps. Dr. Sakharov graduated from high school in 1938 and followed his father into physics. When Hitler's advancing armies threatened the capital in the desperate autumn of 1941, the young Sakharov was evacuated to the Soviet hinterland with thousands of other university students. There, he worked in a munitions factory, made some time-saving innovations that improved the quality of the shells, and received the first of what would become a flood of state awards for his scientific military work. With World War II over, Dr. Sakharov returned to Moscow, married, began raising a family, and joined the prestigious Lebedev Physics Institute. Stalin had ordered a crash program to build an atomic bomb to match the nuclear weapons of the United States, the feared leader of the capitalist West. Inevitably, Dr. Sakhrov was drawn into the frantic effort. In 1948, he joined the teams of scientists who in utmost secrecy were trying to overtake the United States by leapfrogging from uranium fission bombs to build the world's most frightening weapon, the hydrogen fusion bomb. In part because of Dr. Sakharov's contributions, the Soviets chopped development time and began deploying H-bombs much faster than U.S. intelligence had estimated. In 1953, the year Stalin died, Dr. Sakharov, then 32, became the youngest person ever admitted to the prestigious Soviet Academy of Sciences. Bonuses included the Order of Lenin and the Stalin Prize. Three times, he was given the order of Hero of Socialist Labor, and other honors and privileges were his, all awarded in conditions of deepest secrecy. The world outside had no idea of the struggle taking shape between Dr. Sakharov and Nikita Khrushchev over the tempestuous Soviet leader's use of H-bomb tests to pressure the West on political issues. Like other atomic physicists, Dr. Sakharov was acutely aware that radioactive particles circulating in the atmosphere from such blasts could have damaging effects on mankind for centuries to come. He petitioned Khrushchev repeatedly to stop the blasts. Khrushchev, who in his memoirs called Dr. Sakharov "a crystal of morality among our scientists," nevertheless consistently refused to delay or cancel the tests. The refusal to heed his pleas helped activate Dr. Sakharov's conscience. Rebuffed trying to halt an H-bomb test equal to 57 million tons of TNT, the frustrated physicist recalled later, "The feeling of impotence and fright that seized me that day . . . remained in my memory . . . and worked much change in me as I moved toward my present attitude." The political confrontation with the Soviet regime had begun. In less than a decade, Dr. Sakharov had moved far along the path of protest. From the original collection of essays, the physicist began participating in discussions among the Moscow intelligentsia that led to the formation of a series of human rights organizations in the early 1970s. Faced with the regime's stiff-backed opposition to any political dissent, the groups struggled along in near obscurity until the 1975 Helsinki accord, signed by the Soviet Union and more than 30 other nations. Among its provisions were guarantees of freedom of speech, religion, and assembly. Under the Helsinki umbrella, numerous groups sprang into being to monitor the Kremlin's compliance with the guarantees. That same year, Dr. Sakharov was awarded the Nobel Peace Prize for his activism. His first wife, Klavdia, had died of cancer. As his political activism grew, his three children shunned their father; his activities threatened their comfortable existence as privileged offspring of an academician. His colleagues at the Academy of Sciences turned their backs for the same reason. Only some writers and artists of Moscow's intellectual community stood by him, openly signing letters of support from time to time. At the trial of a political activist in 1970, Dr. Sakharov met Bonner, who had been active in dissent for years. Her father, an Old Bolshevik, had been executed in the purges, and her mother had survived 18 years in the Gulag. The physicist and the activist fell in love and soon were married. He moved to her apartment in central Moscow, which became a mecca for abused and disaffected Soviet citizens. Dr. Sakharov and Bonner became familiar figures at political trials of Soviet citizens facing prison on trumped-up charges of slandering the Soviet state. They traveled thousands of miles around the country to remote courts and prisons in support of "those who think differently," as dissidents are known in the Russian language. When the regime expelled dissident author Alexander Solzhenitsyn in 1974, there was no other voice in the land to equal Dr. Sakharov's for strength, range and impact against the regime's totalitarian insistence on ideological conformity. Because he had participated in military projects, the state made clear, it would never consider similar expulsion of Dr. Sakharov, one of the country's greatest scientists. Instead, the regime tried to intimidate the physicist into silence by vilifying him in the controlled press. For example, a major deununciatory campaign erupted against Dr. Sakharov in 1973. Many Western diplomats in Moscow believed it was a prelude to charging him with political crimes against the state. But when Western scientists, led by the Americans, made clear that any punishment of the brilliant physicist would have severe repercussions, the regime backed off. Banned from access to any advanced laboratories and stripped of all his scientific awards when he was exiled to Gorky, Dr. Sakharov turned his inquisitive physicist's mind toward the cosmos. For some years before his death, when he was not engaged in political confrontations, he devoted himself to seeking new understandings of the nature of the universe. In time, Dr. Sakharov drew close to Bonner's children by her first marriage. They had immigrated to the United States in 1978. In 1981, Dr. Sakharov and his wife began a hunger strike while in exile in Gorky to force Soviet authorities to grant an exit visa to Lisa Alexeyeva, the fiancee of Bonner's son, Alexei Semyonov. The fast was ended after 13 days when authorities forcibly hospitalized the Sakharovs and permitted Alexeyeva to leave. This traumatic confrontation, headlined around the world, was repeated in May 1984 when Dr. Sakharov began a new hunger strike to force the authorities to grant his ill wife permission to leave the country for Western medical treatment. Freed from exile in 1986 and even allowed to travel to the United States and other western countries in the last several years, Dr. Sakharov never regained robust health. In addition to his wife, Dr. Sakharov's survivors include three children by his first marriage, Tatiana, Lyubov and Dmitri; and two stepchildren, Tatiana Yankelevich and Alexei Semyonov, both of Newton, Mass.