Grand Ayatollah Hossein Ali Montazeri, Iran's most senior dissident cleric, has died, official media has reported.
Montazeri, 87, was an architect of the 1979 Islamic revolution who fell out with the present leadership.
He had been held under house arrest for several years.
"Hossein Ali Montazeri passed away in his home last night," the official IRNA news agency said on Sunday.
He lived in the city of Qom, which lies south of Tehran, and was referred to as the spiritual leader of the opposition after the country's recent disputed election.
Ghanbar Naderi, an Iran Daily journalist, told Al Jazeera: "This is huge blow to the reformist camp, because he is unreplaceable and nobody is happy to hear about his sad demise.
"He used to say that religion should be separated from politics, because in this way, we can keep the integrity of religion intact."
In August, Montazeri described the clerical establishment as a "dictatorship", saying that the authorities' handling of street unrest after the disputed election in June "could lead to the fall of the regime".
"I hope the responsible authorities give up the deviant path they are pursuing and restore the trampled rights of the people," he wrote.
"I hope authorities ... have the courage to announce that this ruling system is neither a republic nor Islamic and that nobody has the right to express opinion or criticism," Montazeri said in August this year.
Alireza Ronaghi, Al Jazeera's correspondent in Tehran, said that Montazeri's statements were significant because he was once in line to succeed the late Ayatollah Khomeini as Iran's supreme leader.
But Seyed Mohammad Marandi, a political analyst at the University of Tehran, told Al Jazeera in August that Montazeri has been "saying the same thing for around 25 years".
"He is not a major player and has always been very critical," Marandi said.