A supporter of defeated Iranian presidential candidate Mousavi lights candles as she attends a rally in Tehran June 18, 2009. [Agencies]

The Guardian Council, an unelected body of 12 clerics and Islamic law experts close to Khamenei, has said it was prepared to conduct a limited recount of ballots at sites where candidates claim irregularities. But Mousavi says the council supports Ahmadinejad, and he has demanded an independent investigation and a new election.

The ruling clerics still command deep public support and are defended by Iran's most powerful military force - the Revolutionary Guard - as well as a vast network of militias.

But Mousavi's movement has forced Khamenei into the center of the escalating crisis, questioning his role as the final authority on all critical issues.

Khamenei is scheduled to lead Friday's prayers, though it is unclear what he will say, if anything, about Mousavi and the demonstrations. At least one candidate who ran against Ahmadinejad, reformist Mahdi Karroubi, has said he will attend the service at Tehran University.

It was not known if Mousavi or Ahmadinejad would be there, although the president normally attends Friday prayers when Khamenei leads them.

Shortly after the election, Mousavi appealed for the backing of clerics in the holy city of Qom, Iran's seat of Islamic learning and a critical political base for the theocracy. He received shows of solidarity from several liberal ayatollahs but has not captured widespread support.