Iran's most senior dissident cleric, Grand Ayatollah Hossein Ali Montazeri, who emerged as the spiritual father of its reform movement, died on Sunday. He was 87.

For years, Montazeri had accused the country's ruling Islamic establishment of imposing dictatorship in the name of Islam, and he persisted with his criticism following June's disputed presidential election.

His stance made him a hero to the opposition, and his criticisms were even more stinging because of his status. Montazeri had once been designated to succeed Ayatollah Ruhollah Khomeini, the late founder of Iran's 1979 Islamic revolution, as the supreme leader — but the two had a falling out a few months before Khomeini died of cancer in 1989.

Montazeri's grandson, Nasser Montazeri, said he died in his sleep overnight.

The Web site of Iranian state television cited doctors as saying Montazeri had suffered from asthma and arteriosclerosis.

Several top pro-opposition ayatollahs gathered at Montazeri's house after his death, the Gooya News Web site reported.

The official IRNA news agency issued a two-line report on Montazeri's death without mentioning his title and state radio and television broadcasters were equally terse, reflecting the deep tension between the government and its opponents.

Past deaths of high-ranking religious figures were accompanied by wide coverage in state media, along with the broadcast of condolence messages by Iranian leaders to their families and followers.

Montazeri was one of just a few Grand Ayatollahs — the most senior theologians of the Shiite Muslim faith.

After the disputed election, pro-government figures tried to reduce Montazeri's impact by spreading reports that he had become senile and that his supporters were issuing opinions in his name.

Two decades ago, Iran's current Supreme Leader, Ayatollah Ali Khamenei, succeeded Khomeini and has been the target of escalating criticism by Iran's opposition movement since June's disputed presidential vote.

In 1997, Montazeri was placed under house arrest in Qom, 80 miles south of Tehran, after saying Khamenei wasn't qualified to rule.

The penalty was lifted in 2003, but Montazeri remained defiant, saying the freedom that was supposed to follow the 1979 revolution never happened.

After he was placed under house arrest, state-run media stopped referring to Montazeri by his religious title, describing him instead as a "simple-minded" cleric. Any talk about Montazeri was strongly discouraged, references to him in schoolbooks were removed and streets named after him were renamed.

Montazeri was still respected by many Iranians, who observed his religious rulings or supported his calls for democratic change within the ruling establishment.

On Saturday, after months of denials, Iran acknowledged that at least three people detained in the country's postelection turmoil were beaten to death by their jailers.

The surprise announcement by the hard-line judiciary confirmed one of the opposition's most devastating and embarrassing claims against authorities and the elite Revolutionary Guard forces that led the crackdown after the vote in June.