Montazeri, an architect of Iran's 1979 Islamic Revolution who is considered the spiritual father of the present reform movement, is to be buried on December 21 in the Shi'ite holy city of Qom.
Many Iranians, including political and religious figures, are traveling to Qom to attend the funeral.
According to unconfirmed reports, opposition supporters gathered in the squares of the capital Tehran on December 20 to mourn the loss and riot police were present in various parts of Qom.
Iranian news websites are also reporting that opposition leaders, Mir Hossein Musavi and Mehdi Karubi, are both planning to attend the funeral. They have called for a day of mourning.
Iranian Supreme Leader Ayatollah Ali Khamenei has expressed his condolences, but the state news agency IRNA branded Montazeri the "clerical figure of rioters" and dropped his title of grand ayatollah in its early reports.
During his lifetime, Montazeri was transformed from a founder of the Islamic Revolution to a harsh critic of its clerical establishment.
Reformist cleric Mohammad Taghi Fazel Meibodi tells RFE/RL's Radio Farda that Montazeri -- one of the world’s highest Shi’ite authorities -- was regarded as a great defender of human rights in Iran.
“He was a ‘Shi’ite source of emulation’ whose beliefs extended well beyond the common practice of the religious schools. Particularly the issue of human rights was of a great importance for him and he used to read and follow the latest global developments of the issue," Meibodi says.
"He had also written a book about the civil rights, in which he emphasized on the personal rights of every single citizen. I have not seen a Shi’ite grand ayatollah that has such knowledge of human rights.”
Montazeri had been designated to succeed Supreme Leader Ayatollah Ruhollah Khomeini, the late founder of the Islamic Republic.
But the two fell out over Iran's human rights record and the conduct of the Iran-Iraq war a few months before Khomeini died of cancer in 1989.
(LISTEN: RFE/RL's Radio Farda broadcaster Mehrdad Mirdamadi Khouzani speaks about Ayatollah Montazeri's life and legacy.)
With Montazeri forced aside, Iran's current supreme leader, Ayatollah Khamenei, succeeded Khomeini instead.
Montazeri has long been a critic of Khamenei, frequently criticizing his religious qualifications, tyrannical rule, and reliance on security forces.
According to Radio Farda broadcaster Mehrdad Mirdamadi Khouzani, Montazeri has always stuck to Islamic values -- against pragmatism.
"Ayatollah Montazeri always mentioned this -- that Islamic teachings are to be followed even if they are against us when we are trying to implement power on people," Khouzani says.
In 1997, Montazeri famously clashed with Khamenei, whom he outranked in the religious hierarchy, suggesting that he was unqualified to be supreme leader. This led to the closure of his religious school and an attack on his office in Qom. He was placed under house arrest for five years.
Despite political isolation, Montazeri remained an inspiration for Iranian reformists and a respected religious figure.
Montazeri was viewed by some reformist figures as the spiritual father of the opposition Green Movement, which emerged following the country's highly contentious presidential election in June.
Despite his old age and failing health, Montazeri backed the opposition's claims that the election result, which gave President Mahmud Ahmadinejad a landslide victory, had been widely rigged.
In August, Montazeri described the clerical establishment as a "dictatorship," saying the authorities' handling of street unrest following the presidential poll "could lead to the fall of the regime."
Khouzani says he believes that even after his death, Montazeri will continue to influence Iran’s opposition movement.
"I think that no one can replace him in terms of religious authority and also political background. But he has been so open minded and he has written so many essays and books and made several speeches criticizing the fundamental elements of the Islamic regime that I believe that his thoughts still would give guidance to his followers," Khouzani said.