Tens of thousands of Iranians are gathered in the holy city of Qom for the funeral of dissident cleric Grand Ayatollah Hossein Ali Montazeri.
Many of this number are opposition supporters, and analysts say that this could be the catalyst for renewed protests.
Ayatollah Montazeri, aged 87, was critical of the government of president Mahmoud Ahmadinjad and said that his re-election was a fraud.
The funeral is a demonstration that the authorities cannot forbid. However security on the ground is said to be tight and there have been reports of arrests.
The opposition website, Kalme, reported that a bus carrying opposition supporters to Qom was stopped and some of those on board arrested.
Foreign media have been banned from covering the funeral ceremony.