Montazeri urged Iran's clerics to "be in step with the people"
Grand Ayatollah Hoseyn Ali Montazeri, who has died aged 87, was one of the leaders of Iran's 1979 Islamic Revolution and at one stage he was designated to become its Supreme Leader.
But after falling out with the Islamic Republic's founder, he became an increasingly vocal critic of the regime, accusing it of imposing dictatorship in the name of Islam.
Following Iran's disputed presidential election in June this year, when President Mahmoud Ahmadinejad won after widespread alleged vote-rigging, Montazeri again criticised Iran's rulers.
Montazeri - seen by many as one of Shia Islam's most respected theologians - even issued a fatwa condemning Mr Ahmadinejad's government.
In a recent interview with the BBC, he told Iran's clerics that they "can and must" act to bring about reform in the country.
He urged them to "be in step with the people" and tell them about their rights.
He repeatedly warned that Tehran's handling of mass street rallies after the election "could lead to the fall of the regime".
Ayatollah Montazeri was born in 1922 to peasant parents in the city of Najafabad, south of Tehran.
MONTAZERI BBC INTERVIEWS
Exchanged emails detailing his thinking on reform in October 2009
Interviewed at his home in Qom in June 2008
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He studied theology in Isfahan and later continued his education in Qom under the Islamic Republic's founder, Ayatollah Ruhollah Khomeini.
He later became active in the anti-Shah movement and was jailed in 1974. He was released in 1978 - a year before the Islamic Revolution led by Khomeini.
While Khomeini was in exile, Montazeri was nominated as his representative in Iran.
Soon after the revolution Montazeri was marginalised for openly questioning decisions taken by Khomeini and calling for a transparent assessment of the new regime's failures.
In 1997, Montazeri challenged the powers of Ayatollah Ali Khamenei - who succeeded Khomeini as the Supreme Leader after his death in 1989.
This led to the closure of Montazeri's religious school, an attack on his office in Qom and to a period of house arrest.
After his release Ayatollah Montazeri, who as a scholar ranked higher than Khamenei, remained a dissident voice from within the heart of the clerical elite.
He repeatedly stated that his opinions were guided by his "sense of religious duty".
In recent years, Iran's state-run media stopped referring to Montazeri by his religious title, describing him instead as a "simple-minded" cleric.
The authorities also strongly discouraged any talk about him, removing references to him in school textbooks.