Thich Huyen Quang, the patriarch of an outlawed Buddhist church in Vietnam who spent more than two decades in and out of house arrest, died after months of ailing health. He was 87.

The leader of the Unified Buddhist Church of Vietnam died Saturday of multiple organ failure a day after being transferred from a hospital to his monastery at his request, a spokeswoman of the International Buddhist Information Bureau in Paris, which speaks for the outlawed church, Penelope Faulkner said.

An outspoken proponent of religious freedom and human rights, Quang had long been confined to the Nguyen Thieu Monastery in the southern province of Binh Dinh.

“He was a real pioneer, and that’s why Vietnam kept him isolated and they wanted to keep him out of the way,” she said. “He kept determined to the very end.”

The church’s deputy leader, Thich Quang Do, 80, broke out of house arrest at his monastery in Ho Chi Minh City to be at Quang’s side when the patriarch was hospitalized, Ms. Faulkner said. Mr. Do held a prayer service after Quang’s death and plans to oversee a funeral scheduled for next week, she said.

Buddhist monk Thich Minh Tuan said Quang’s followers were preparing a “simple but solemn funeral” and he will be buried at the pagoda.

“He passed away very peacefully with many of his followers at his bedside,” Mr. Tuan said.

State-controlled press over the past few days have accused Mr. Do, a Nobel Peace Prize nominee, and other senior members of the banned church of attempting to use Quang’s death for “personal political gains.”

The Buddhist sect was effectively banned in 1981 when it refused to merge with the state-sponsored Buddhist Church of Vietnam.

Vietnam’s communist government allows only a handful of officially approved religious groups to worship, outlawing all other sects.