Tehran, Iran (CNN) -- Senior Iranian dissident cleric Grand Ayatollah Hussein Ali Montazeri died overnight, several state-run Iranian news agencies reported Sunday.

His son, Ahmad, told the official IRNA news agency that his 87-year-old father passed away due to cardiac arrest at 1:30 a.m. on Sunday morning, Press TV reported. The semi-official ILNA agency said he died at his home in the Iranian holy city of Qom.

Montazeri -- a key figure in Iran's Islamic Revolution 30 years ago -- was perhaps the most prominent cleric who publicly criticized the presidential elections last June that returned hard-liner Mahmoud Ahmadinejad to the presidency.

On his Web site, Montazeri described the street protests that followed the polling as a challenge to the "very legitimacy of the Islamic Republic."

His death is "a terrible blow for the (reformist) green movement and everybody who has been active these past few months," said Mehrdad Khonsari, a senior research consultant at the Centre for Arab and Iranian Studies in London.

In the last six months, Montazeri had become the voice of the green movement on key foreign policy issues, Khonsari said. The cleric was able to comment more freely than reformist presidential candidates.

"He was a very senior grand ayatollah with very good revolutionary credentials, and a very vocal critic of the ruling establishment," Khonsari said.

"He was a major thorn in the back of (Iranian Supreme Leader Ayatollah Ali) Khamenei. He was a bastion for all the people opposing the excesses of the system," even within the ruling establishment, he said.

Montazeri, for example, took a public stand against nuclear weapons in October, issuing a fatwa, or religious edict, declaring them to be "un-Islamic" and instructing Muslims to take the lead in banning them, said Nazenin Ansari, diplomatic editor of the weekly newspaper Kayhan in London.

Iran's nuclear program has been the subject of intense international concern, with the United Nations considering expanding sanctions against the Islamic state despite its insistence it does not intend to build a bomb.

Montazeri's upcoming funeral must be a cause for concern for the authorities in Tehran, Ansari said, but because of his standing, they cannot ban it.

"Already there are people starting to march towards his home," she said she had heard from sources inside Iran. "He has a very large public following especially since the June elections and all his statements in favor of human rights."

It is not clear whether his death sparked demonstrations, as some reports said.

CNN producer Shirzad Bozorgmehr saw no unusual activity in the vicinity of Tehran University, at Haft-e-Tir Square or Revolution Square in Tehran on Sunday.

Montazeri's Web site called for a funeral procession from his home to the Massoumeh Shrine in Qom, the holy city where he lived, starting at 9 a.m. on Monday (12:30 a.m. ET).

Any demonstrations resulting from the burial could spill over into the capital, Ansari said.

"Qom is very close to Tehran and this is a major event," she said, adding that it is difficult to predict what would happen. "It would be questionable how a lot of the foot soldiers of the revolution would react."

The timing of his death could also be significant, she said, since it comes in the month that first saw the demonstrations that spiraled into the revolution that toppled the monarchy and brought the Islamic regime to power.

Montazeri was once a confidant of Ayatollah Ruhollah Khomeini, who led Iran for a decade after the revolution.

He studied under Khomeini and then acted as his representative in Iran after Khomeini was exiled by the shah for criticizing the monarchy, Ansari said. Montazeri, too, was jailed and exiled.

After the revolution, he was the heir apparent to Khomeini, and played a key role in writing Iran's constitution as the first chair of the powerful Guardians Council. He was also the leader of Friday prayers in Tehran, one of the most public and influential pulpits in the country.

But he was supplanted by the current supreme leader, Ayatollah Ali Khamenei, and was placed under several years of house arrest in 1997.

"His problems with Khomeini started in the summer of 1988 when they started executing... thousands of leftist prisoners," Ansari said. "He never was in agreement with that, and then he started to fade" from power.

"Once Khamenei became leader, Montazeri objected," she said.

Montazeri said Khamenei "was elevated above his (religious) qualifications," pointing out that he had not earned the title of ayatollah. "It was then he was arrested."

Recently he has objected to the current regime's characterization of his one-time mentor Khomeini as a saint, she added.

And following the disputed elections in June, "he issued a ground-breaking fatwa articulating the policies of the (reformist) green movement to the world. "Given the dynamics of the political situation in Iran, the regime will have difficulty controlling his funeral and where it might lead," she said.

Montazeri had been equally critical of the parliamentary electoral process in 2004, arguing in favor of reformist candidates.

Sitting behind his desk with a government minder in the room, he said then that Iran's Islamic revolution had lost its way.

"Even I, who used to be a leading figure in the revolution, have not the right to speak out," he said. "Authoritarianism will never last long. The gentlemen in power must submit to the wishes of the people, or they will be swept away."