He suffered for many years from diabetes, a heart condition and high blood pressure, said the International Buddhist Information Bureau in Paris, which speaks for the outlawed church and announced the death.
Mr. Do received awards for his activism, including the Rafto Prize for human rights and the Hellman/Hammett award, which the New York-based group Human Rights Watch gives to writers for courage in the face of political persecution.
“People are very afraid of the government,” Mr. Do told the Associated Press in a rare 2003 interview. “Only I dare to say what I want to say. That is why they are afraid of me.”
Even as Vietnam has embraced economic liberalization and free markets, its political system remains firmly under the control of the communist government.
Mr. Do said that freedom, democracy and human rights “are more important than economic development” and that without them, “we cannot make any progress in the real sense.”
He had been under near constant surveillance for years at his home in Ho Chi Minh City, the Thanh Minh Zen Monastery, where according to his supporters he organized microcredit programs and flood relief campaigns while coordinating provincial committees of his outlawed church.
According to the International Buddhist Information Bureau, he had been deprived of all means of communicating independently for the past year after he moved to the city’s Tu Hieu Pagoda, after being sent out of the Thanh Minh Zen Monastery and briefly living in northern Vietnam.
“The people who looked after him confiscated his cellphone and prevented his personal assistant from visiting him,” the Paris-based support group said in an email.
Buddhism is the primary religion among fast-growing Vietnam’s 98 million people, although there are also millions of Christians. The government has become more tolerant of public worship in recent years but allows only a handful of officially approved religious groups.
Mr. Do was born Dang Phuc Tue in northern Thai Binh province on Nov. 27, 1928. His defiance of repressive governments predated the 1975 communist takeover of U.S.-backed South Vietnam and the former Saigon, now Ho Chi Minh City.
He was first imprisoned in 1963 under Catholic leader Ngo Dinh Diem, and after Vietnam was reunified, he protested against its ruling communists.
After his 1977 arrest on charges of “undermining national solidarity” and conducting “anti-revolutionary activities,” Mr. Do endured nearly two years of solitary confinement in a roughly three-by-six-foot prison cell, gazing through a window the size of his hand until international pressure forced his release, his supporters say.
In 1981, the government created the Communist Party-controlled Vietnam Buddhist Church and forced Mr. Do into internal exile in Thai Binh province. He was later offered the leadership of the official church, his supporters say, but he refused and in 1992 fled to Ho Chi Minh City.
In 1995, he was sentenced to five years in prison on charges that included sending two faxes to overseas Buddhists accusing the government of obstructing a church-sponsored flood relief mission. International pressure led to his early release in 1998, but he was again placed under house arrest in 2001.
Although Mr. Do was officially freed two years later, a 2005 report by the United Nations Working Group on Arbitrary Detention cited an unnamed source as saying that restrictions on Mr. Do were “equivalent to detention.”
Over the years Vietnam denied accusations that it placed Mr. Do and a former leader of the Unified Buddhist Church of Vietnam, the late Thich Huyen Quang, under house arrest. They “lead normal lives” at their respective monasteries, Foreign Ministry spokesman Le Dung said in 2005.
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