Qom , Iran ( CNN ) -- Tens of thousands of people chanted anti-government slogans Monday before the funeral of a top Iranian cleric who turned into one of the regime 's most vocal critics , a witness said . They shouted `` death to the dictator ! '' outside the home of senior Iranian dissident Grand Ayatollah Hussein Ali Montazeri , who died early Sunday . He was 87 . 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 . Protesters and mourners began to gather near his home in this holy city around 7 a.m. , swelling to huge numbers by the time the funeral procession began , said the witness . Both uniformed and plain clothes security forces turned out for the funeral , with the undercover officers arriving in several buses and mingling with the crowds . The witness did not see any clashes but said tensions were high as protesters stood and chanted just steps away from security forces . Many of the protesters and mourners had driven to Qom , where Montazeri lived , from Tehran and other cities hundreds of miles away , the witness said . Mourners from Najafabad , Montazeri 's hometown , passed out dates , an Iranian tradition at funerals . The night before the funeral , chants of `` God is great '' -- which the opposition uses as a rallying cry -- could be heard in several Tehran neighborhoods . The chants were louder than usual , witnesses said . Around 300 to 400 people gathered at Tehran 's Mother 's Square and lit candles on Sunday night in memory of Montazeri , several sources told CNN . The witnesses did not see any clashes between security forces and the people at the gathering . CNN is not naming its sources to protect their security . Writing on his Web site after the disputed June presidential elections , 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 . `` 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 . 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 . 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 . `` He has a very large public following especially since the June elections and all his statements in favor of human rights , '' Ansari said . 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 ) . Demonstrations at 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 . 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 . '' CNN 's Reza Sayah and Richard Allen Greene contributed to this report .
Thousands chant anti-government chants at cleric 's funeral . Senior Iranian dissident cleric Grand Ayatollah Hussein Ali Montazeri died Sunday . Montazeri was one of several prominent clerics who publicly criticized the presidential elections last June . Montazeri was once heir apparent to late Ayatollah Ruhollah Khomeini , architect of Iran 's 1979 Islamic revolution .