Thousands of mourners turned out in Iran's holy city of Qom on Monday for the funeral of top dissident cleric Grand Ayatollah Hossein Ali Montazeri, opposition websites reported.
Montazeri died late on Saturday aged 87. Montazeri was named in the 1980s to succeed revolutionary leader Ayatollah Ruhollah Khomeini as Iran's top authority, but fell out with the leadership.
Montazeri 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 his authority.
Montazeri, one of the chief architects of the Islamic republic, was a student and close ally of Khomeini, whom he was set to succeed.
Clash flared when security forces around Montazeri's house clashed with stone-throwing protesters, the reformist website Norooz said. There was no immediate official comment.
But Reuters news agency says could not verify the repor independently. It says it is due to foreign media banned from reporting directly on protests and were told not to travel to Qom for Montazeri's funeral.
The reformist website Jaras said "hundreds of thousands of people" joined a procession for Montazeri, an architect of Iran's 1979 Islamic revolution that toppled the U.S.-backed shah.
"Innocent Montazeri, your path will be continued even if the dictator should rain bullets on our heads," they chanted.
Riot police were out in force in Qom, 125 km (80 miles) south of Tehran.
The reformist Kaleme website said crowds had shouted: "Today is the day of mourning and the green Iranian nation is the owner of this mourning," referring to the opposition colour.
Mourners carried pictures of Montazeri and of another Qom-based dissident cleric, Grand Ayatollah Yusuf Sanei.
Opposition figures declared a national day of mourning for Montazeri, who was named in the 1980s to succeed revolutionary leader Ayatollah Ruhollah Khomeini.
Khamenei, who succeeded Khomeini after he died in 1989, expressed his condolences, but said he asked God to forgive Montazeri over a "difficult and critical test" that he faced towards the end of Khomeini's life, ISNA news agency said.
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Khomeini's grandson, Hassan Khomeini, a cleric, paid tribute in his condolence message to a man he said had spent many years of his honourable life on the path of advancing the high goals of Islam and the Islamic revolution, ILNA news agency reported.
Human rights activist and Nobel Prize laureate Shirin Ebadi called Montazeri "the father of human rights in Iran".
In the wake of the street protests that followed the election dispute, Montazeri was referred to as the spiritual leader of the opposition.
In August, Montazeri described the clerical establishment as a "dictatorship", saying that the authorities' handling of street unrest after the disputed re-election of Mahmoud Ahmadinejad, the president, "could lead to the fall of the regime".