Top Iranian dissident cleric Grand Ayatollah Hossein Ali Montazeri has died, his office said on Sunday, stirring calls for opponents of the hardline Islamic regime to gather on the streets of Tehran. Montazeri, 87, a fierce critic of President Mahmoud Ahmadinejad, died of a natural illness on Saturday night and is set to be buried on Monday, said his office in the holy city of Qom, where he had been based for several years. 'He was diabetic and had been using insulin for years ... He had also some lung problems and asthma. In fact he was suffering from several diseases,' his doctor told state television. The grand ayatollah was an inspiration to rights advocates and pro-reform groups and was considered by his followers as the highest living authority of Shiite Islam in Iran. According to the opposition website Rahesabz.net, opposition supporters called for a gathering to mourn the cleric at Tehran's Mohseni square later on Sunday, adding that 'sporadic gatherings' were already seen in the capital. Meanwhile, Internet connection slowed to a crawl, as has been the case whenever the authorities anticipate opposition demonstrations. Montazeri is to be buried in the shrine of Masoumeh, a revered Shiite figure, in Qom, his office told selfnews. Foreign media have been banned from covering the ceremony. Once designated as the successor to the founder of the 1979 Islamic revolution, Ayatollah Ruhollah Khomeini, Montazeri came out in bold support of the Iranian opposition when it rejected the re-election of Ahmadinejad in June. Iran's supreme leader Ayatollah Ali Khamenei offered condolences to his family although Montazeri was also critical of him and questioned his credentials for being the country's highest religious authority. 'He was an accomplished theologian and a prominent teacher who spent a large part of his life for Imam's (Khomeini's) cause,' Khamenei said in a statement carried by state television's website. He also asked divine forgiveness for Montazeri over a 'difficult ordeal' that the late cleric had undergone, alluding to his fallout with Khomeini. The cleric had long been critical of the concentration of power in the hands of the supreme leader and called for changes to the constitution which he helped draw up after the 1979 Islamic revolution, to limit the leader's authority. Montazeri had often criticised hardliner Ahmadinejad over his domestic and foreign policies, including Tehran's nuclear standoff with the West. He had also called on other leading clerics to break their silence over incidents and rights abuses during the government's crackdown on opposition supporters protesting the presidential election, which they charge was massively rigged in Ahmadinejad's favour. Controversially, he had called for direct talks between Tehran and Washington to avert conflict over Iran's controversial programme of uranium enrichment. The grand ayatollah described the 1979 seizure of the US embassy in Tehran a 'mistake' even though he said he had approved of the move at the time. Montazeri, one of the main architects of the Islamic republic, was a student and close ally of Khomeini, whom he was set to succeed. But the cleric fell from grace in the late 1980s after he became too openly critical of political and cultural restrictions, most notably Iran's treatment of political prisoners and opposition groups. Montazeri resigned months before Khomeini's death in 1989, and was told by Khomeini to stay out of politics and focus instead on teaching in the city of Qom. Unfazed by such warnings, he continued to speak out. The grand ayatollah also questioned the theological credentials of Khamenei. This was branded as treason, and in 1997 he was placed under house arrest. Freed after five years on health grounds during the reformist presidency of Mohammad Khatami, the grand ayatollah vowed that he would continue to speak out in defence of freedom and justice. In his latest reaction to the post-vote crackdown on protests, Montazeri strongly slammed 'the killing of innocent people, the arrest of political activists and freedom-seekers as well as their illegal show trials.' Iran's state news agency IRNA branded him as the 'clerical figure of rioters' -- the term used by pro-government media for post-vote protesters -- and dropped his clerical title of ayatollah in its early reports. 