Thich Quang Do, the patriarch of the banned Unified Buddhist Church of Vietnam and a defiant champion of religious freedom, democracy and human rights in his country, died on Saturday. He was 91.

His death was confirmed by the Paris-based International Buddhist Information Bureau, an arm of his church. There was no information on where he died. A spokeswoman for the organization said that the Vietnamese authorities had held him incommunicado at the Tu Hieu Pagoda in Ho Chi Minh City and that it had been impossible for members of his organization to make contact with him. He had diabetes and a heart condition for many years, she added.

Thich Quang Do had for decades repeatedly challenged, and angered, the Communist government on issues of religious and political freedom and had effectively been under house arrest since 2003. He had spent the last 30 years or so in and out of prison, under house arrest or forced into internal exile for refusing to submit the Unified Church to government control.

He issued a stream of public statements over the years, putting him in the forefront of religious activism in Vietnam, which permits only a single government-sanctioned Buddhist organization. The Unified Church, founded as an umbrella organization for various Buddhist sects in 1964, was banned.

His themes were as much secular as religious, echoing some of the main concerns of political dissent in Vietnam.

One such statement, delivered in a video message to the United Nations in 2005, amounted to a political manifesto.

“Without democracy and pluralism we cannot combat poverty and injustice nor bring true development to our people,” the statement said. “Without democracy and pluralism we cannot guarantee human rights, for human rights cannot be protected without the safeguards of democratic institutions and the rule of law.”

In 2001, Thich Quang Do published “Appeal for Democracy in Vietnam,” an eight-point declaration calling for a multiparty system, free elections, independent trade unions and the abolition of “all degrading forms of imported culture and ideologies that pervert Vietnamese spiritual and moral values.”

He was instrumental in forging links between dissidents in the north and south, ending a decades-long geographical and ideological divide. As well, he was a respected scholar with more than a dozen published works, including novels, poetry, translations and studies of Vietnamese Buddhism.

Thich Quang Do received a number of human rights awards, including Norway’s Rafto Prize, which cited “his personal courage and perseverance through three decades of peaceful opposition against the Communist regime in Vietnam.”

In 1978, he and Thich Huyen Quang, the patriarch of the Unified Church at the time, were nominated for the Nobel Peace Prize by the Irish peace activists Betty Williams and Mairead Corrigan Maguire, the 1976 laureates.

The Commission on International Religious Freedom, an independent body established by Congress, spoke out on Thich Quang Do’s behalf in 2018. Its vice-chairwoman at the time, Kristina Arriaga, said, “I urge the government of Vietnam to respect his freedom of movement and freedom to reside wherever he chooses.”

Thich Quang Do was born Dang Phuc Tue, on Nov. 27, 1928, in Thai Binh Province, in northern Vietnam. He assumed the Dharma name Thich Quang Do after becoming a monk at the age of 14. Thich is an honorary family name used by monks and nuns.

He said his life’s course was set at the age of 17, when he witnessed the execution of his religious master, Thich Duc Hai, by a Communist revolutionary tribunal. “Then and there I vowed to do all that I could to combat fanaticism and intolerance and devote my life to the pursuit of justice through the Buddhist teachings of nonviolence, tolerance and compassion,” he wrote in 1994, in an open letter to Do Muoi, the general secretary of Vietnam’s Communist Party at the time.

He added: “Little did I realize how that simple vow would lead me down a path paved with prison cells, torture, internal exile and detention for so many years to come.”

He and thousands of Buddhists were arrested in 1963 in a broad crackdown by the government of Ngo Dinh Diem, but he was released a few months later when Diem was deposed and assassinated in a military coup.

The Communist side won the Vietnam War in 1975, and two years later Thich Quang Do was put in solitary confinement for his attempts to organize a nonviolent struggle to protect religious freedom. Beginning in the 1980s he spent a decade in internal exile as punishment for his activism and public statements. His 84-year-old mother was exiled with him, and died in 1985 from malnutrition and inadequate medical care.

In a turnaround, the Communist government in 1990 invited him to take up a post in the state-sponsored Vietnam Buddhist Church, but he refused and continued his opposition.

In April 2006, in the early years of his final term of house arrest, he predicted the ultimate victory of his secular ideals.

“There will come a time when the authorities will be unable to silence all of the people all of the time,” he said. “The moment will come when the people will rise up, like water bursting its banks,” and when that happens, he added, “the situation in Vietnam will be forced to change, and a democratic process will emerge.”

Doan Bao Chau contributed reporting from Hanoi.