BBC: Crowds of mourners are gathering in the Iranian city of Qom following the death of leading reformist cleric Grand Ayatollah Hoseyn Ali Montazeri at 87. BBC News
Crowds of mourners are gathering in the Iranian city of Qom following the death of leading reformist cleric Grand Ayatollah Hoseyn Ali Montazeri at 87.
Some pro-reform websites say thousands of people are travelling to the city ahead of Monday's funeral.
Other unverified reports say opposition supporters are also gathering in some squares in Tehran, fuelling government concern of increased political tension.
Iran faced serious unrest after its disputed presidential election in June.
Grand Ayatollah Montazeri, one of Shia Islam's most respected figures and a leading critic of President Mahmoud Ahmadinejad, himself said in August that the turmoil following the election "could lead to the fall of the regime".
He said Iran's clerical leadership was a dictatorship and issued a fatwa condemning the government after the election.
The BBC's Jon Leyne says his death comes at a crucial time in a standoff between the government and opposition, and the leaders will fear a big turnout for Monday's funeral.
It is scheduled to start at 0900 local time (0530 GMT) and it is reported that no foreign media will be allowed to attend. Reporting in Iran by foreign organisations has been restricted since the post-election protests.
One reformist website, Jaras, has quoted opposition leaders Mir Hossein Mousavi and Mehdi Karroubi as calling for Monday to be a "day of national mourning" and urging people to attend the funeral.
Large crowds have already gathered outside the grand ayatollah's home in Qom, 125km south (90 miles) of Tehran, following his death on Saturday evening.
The moderate Parlemannews website said that thousands of people were travelling from all over the country to be at the funeral.
Grand Ayatollah Montazeri died of natural causes, his family said.
He will be laid to rest at the shrine of Hazrate Masoumeh, one of the most revered female saints in Shia Islam, his office told AFP news agency.
Another moderate website, Tagheer, which backs Mehdi Karroubi, said it had reports of people gathering in the Mohseni and other squares in Tehran to mourn. The reports have not been independently verified.
Tagheer also said there were reports of heavy riot police presence in Qom.
However, Grand Ayatollah Montazeri's grandson, Naser Montazeri, told Reuters news agency: "People and friends are coming to express their condolences but there are no special security measures around our house."
State news agency Irna did not use the ayatollah title in its early reports of Grand Ayatollah Montazeri's death and referred to him as the "clerical figure of rioters".
Hoseyn Ali Montazeri was a moving spirit in the 1979 revolution which created Iran's Islamic state.
But during his lifetime, the cleric was transformed from a pillar of the revolution to one of the most vocal critics of its leadership.
He repeatedly accused the country's rulers of imposing dictatorship in the name of Islam and said the liberation that was supposed to have followed the 1979 revolution never happened.
Hoseyn Ali Montazeri was one of the early backers of the man who became the Islamic Republic's founder, Ayatollah Ruhollah Khomeini.
Hoseyn Ali Montazeri organised public protests in support of Khomeini and was repeatedly detained and tortured in jail during the years of the monarchy.
Hoseyn Ali Montazeri was designated to succeed Khomeini, but the pair fell out over Iran's human rights record a few months before Khomeini died of cancer in 1989.
In 1997 he famously clashed with Khomeini's successor, Ayatollah Ali Khamenei, whom he outranked in the religious hierarchy, after questioning the powers of the Supreme Leader.
This led to the closure of Grand Ayatollah Montazeri's religious school and an attack on his office in Qom. He was placed under house arrest for six years.
After his detention, state-run media began referring to him as a "simple-minded" cleric, references to him in schoolbooks were erased and streets named after him were renamed, but he remained defiant.
In his opposition to President Ahmadinejad, he became an unlikely inspiration for Iranian reformists.
Despite his old age and failing health, Grand Ayatollah Montazeri backed the opposition's claims that the 2009 election result, which gave President Ahmadinejad a landslide victory, had been widely rigged.
The cleric had often said his opinions were guided by his "sense of religious duty".
Iran's authorities denied the rigging allegations and said the post-election protests were a foreign-backed plot to bring down the leadership.