TEHRAN, Iran: In apparent defiance of Iran's supreme leader Ayatollah Ali Khamenei, a powerful cleric declared the Islamic Republic in crisis after a disputed election, and tens of thousands of protesters used Friday prayers to stage the biggest show of dissent for weeks.

Clashes erupted in central Tehran between police and followers of opposition leader Mirhossein Mousavi, who still contests official results that showed President Mahmoud Ahmedinejad had been re-elected by a wide margin.

"Police fired teargas and beat supporters of Mousavi in Keshavarz Boulevard," a witness said, adding that protesters were carrying hundreds of green banners -- Mousavi's campaign color -- and chanting "Ahmadinejad, resign, resign."

State television showed footage of police firing tear gas to disperse protesters, chanting "Death to dictator" and "Mousavi we support you."

Former President Akbar Hashemi Rafsanjani, a moderate who backed Mousavi's election campaign, said many Iranians had doubts about the official result of the June 12 vote.

"I hope with this sermon we can pass through this period of hardships that can be called a crisis," said the cleric, leading prayers for the first time since the poll.