Tens of thousands of mourners attended the funeral of Grand Ayatollah Hossein Ali Montazeri, the religious leader of Iran's opposition movement, and many turned it into an anti-government protest, marred by sporadic violence, according to reports from the Shiite Muslim holy city of Qom. Groups of vigilantes clashed with several of the mourners, and both groups threw stones and other objects at each other, witnesses and opposition Web sites reported. The reports could not be independently verified. Authorities denied foreign correspondents permission to travel to Qom, which is about 90 miles south of Tehran. About 100 members of the pro-government Basij militia attacked the house of the late ayatollah and tore up a banner displaying his portrait, his son Saeed Montazeri said in a telephone interview from Qom. 'They attacked, they lost all control,' Montazeri said, calling from the house. 'They started to throw stones at people and tore down the mourning banner of my father.' Montazeri said that after the attack he saw several wounded people. 'The huge crowds in the funeral kept them from taking over,' he said. Opposition Web sites said that hundreds of thousands of people attended the funeral, which took place Monday morning. Several leading ayatollahs paid their respects to Montazeri, who was one of the highest-ranking Shiite clerics in the world but for years lived under house arrest for criticizing political leaders. Opposition leaders Mir Hossein Mousavi and Mehdi Karroubi visited Montazeri's house, according to the opposition Web site Kaleme. Montazeri's body was carried in a shrouded glass coffin on top of a semi-trailer, which moved slowly through seas of weeping people toward the shrine of Hazrat-e Masoumeh, a female Shiite saint. There, the cleric was buried next to his son, who died in the Iran-Iraq war of the 1980s. Young women wearing green headbands, the color of the opposition, could be seen holding portraits of the late ayatollah. 'Many hip young people from Tehran, who usually have no interest in Montazeri, were there, crying,' one witness said. 'It was very moving, and many people shouted slogans against the government.' People shouted political slogans such as 'Montazeri is not dead, the government is dead' and 'Dictator, dictator, we will follow Montazeri's path.' Government supporters, who were present in smaller numbers, shouted, 'Hypocrites leave Qom.' Montazeri's death in his sleep of multiple organ failure Sunday dealt a blow to Iranian human rights and democracy advocates, who considered him their spiritual guide. The death of the ayatollah, who was once designated to lead the Islamic Republic, comes during an already tense Shiite mourning period, called Muharram, which millions will observe by taking to the streets in the coming nights to mourn a revered Shiite saint. After Grand Ayatollah Ali Sistani, an Iranian-born Shiite cleric residing in Najaf, Iraq, Montazeri was considered the highest Shiite authority in the world. A round-faced cleric who wore thick, black, horn-rimmed glasses, Montazeri played a central role during the biggest crises of the 30-year-old Islamic Republic. As one of Iran's main revolutionary clerics supporting the late Ayatollah Ruhollah Khomeini, the leader of Iran's 1979 Islamic revolution, Montazeri helped oust the U.S.-backed Shah Mohammad Reza Pahlavi in 1979 and strongly supported the ideological spread of the Islamic revolution. In 1989, Montazeri, who had been designated to succeed Khomeini as Iran's supreme leader, had a public falling out with the leadership over the mass execution of political opponents. He was banned from public political activities and placed under house arrest in Qom. He returned to the political stage in 1997 with a strongly worded speech challenging the credentials of Khomeini's successor as supreme leader, Ayatollah Ali Khamenei. His criticism prompted demonstrations against him and led to more restrictions on his house arrest, which was quietly lifted in 2003. In July, a month after President Mahmoud Ahmadinejad's disputed re-election sparked riots in Iranian cities, Montazeri issued religious edicts, or fatwas, saying that the supreme leader was unqualified and was working with Ahmadinejad's government against religion. Government supporters, who consider Khamenei to be above criticism, were highly agitated. For the grass-roots opposition movement that has united middle-class Tehran residents, religious figures and ousted politicians against the government after the June election, Montazeri became a religious leader. Many politicians in the opposition regard him as a hero who defended democratic values in the Islamic system. 