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.
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.
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