“Bukovsky’s heroic speech to the court in defense of freedom and his five years of martyrdom in a despicable psychiatric jail will be remembered long after the torturers he defied have rotted away,” the exiled Russian novelist Vladimir Nabokov wrote in 1974.

Galina Ackerman, a Russian émigré writer and translator in Paris who worked closely with Mr. Bukovsky in Resistance International, an anti-Communist organization set up in the 1980s, described him in a phone interview on Monday as “the great thinker of the whole Soviet dissident movement — the most resolute and politically clearheaded.”

Mr. Bukovsky, she said, “had a better vision than anyone of what was needed to weaken the Soviet regime.”

Mr. Bukovsky was freed in 1976 in exchange for Chile’s release of Luis Corvalán Lepe, the head of the Chilean Communist Party at the time, with the United States acting as intermediary. Mr. Bukovsky, who was 33, had been serving a seven‐year prison sentence on charges of anti‐Soviet agitation. The exchange took place in Switzerland, and Mr. Bukovsky then flew to London.

While many exiled dissidents, many of them intellectuals, focused on writing tracts, Mr. Bukovsky lobbied politicians in Britain and the United States to stand firm against Moscow. After the Soviet invasion of Afghanistan in 1979, he pressed for the release of Soviet soldiers taken prisoner so that they could publicize the Soviet army’s brutal tactics.

His stubborn hostility to Soviet power led him to misjudge the Soviet leader Mikhail S. Gorbachev, and he initially dismissed as a ruse Mr. Gorbachev’s liberalizing policies that led to the collapse of the Soviet Union in 1991.

But it also made him deeply aware of the tenacity of anti-democratic forces in Russia, and long before Mr. Putin took power on New Year’s Eve 1999, Mr. Bukovsky had warned that what seemed to be a new dawn of democracy in Russia would not last long. “His skepticism put him far ahead of everyone else,” Ms. Ackerman said.