This photo obtained by The Associated Press outside Iran shows two opposition demonstrators flashing victory sign during a protest in Tehran on Friday. (Associated Press)

Supporters of Iranian opposition leader Mir Hossein Mousavi turned out in the thousands to pack Iran's main prayer hall in Tehran University Friday, in what was a loud show of defiance targeted at the ruling government.

Outside, police and pro-government Basiji militiamen fired tear gas and used batons on some of the protesters, who chanted "death to the dictator" and called on President Mahmoud Ahmadinejad to resign.

Dozens were arrested, piled in trucks and taken away, witnesses said.

Meanwhile, young protesters wearing green bandanas over their mouths took to the streets, despite the tear gas in the air. Some set a bonfire in the street and waved their hands in the air in victory signs.

The clash came as pro-Mousavi supporters gathered inside the university to listen to senior cleric Akbar Hashemi Rafsanjani, who called for the release of those arrested in the protests following the controversial election.

With Mousavi sitting in the front row, Rafsanjani spoke at the prayer hall for the weekly Islamic prayers, considered one of Iran's most important and symbolic political platforms. In recent weeks, Iranian hard-line clerics have used the platform to tell Iranians to accept Ahmadinejad's victory.

"It's not necessary ... to keep individuals in jail," Rafsanjani said. "Let them join their families. We should not let enemies criticize or laugh at us ... for keeping our people in jail," he said.

'Azadi, azadi'

Rafsanjani said hardliners should show sympathy for those arrested, and criticized the crackdown.

In this photo released by the Hayat News Agency, Ayatollah Akbar Hashemi Rafsanjani, delivers a Friday prayer sermon at Tehran University on Friday. (Hayat News Agency/Meisam Hosseini/Associated Press) Worshippers interrupted Rafsanjani with chants of "azadi, azadi" — Persian for "freedom" — and the cleric got tears in his eyes as he spoke of how Islam's Prophet Muhammad "respected the rights" of his people.

"Sympathy must be offered to those who suffered from the events that occurred and reconcile them with the ruling system. This is achievable. We need to placate them."

He urged unity and said doubts about the June 12 election results must be cleared up.

"There are two currents. One doesn't have any doubt and is moving ahead with their job. And there are a large portion of the wise people who say they have doubts. We need to take action to remove this doubt."

Weeks after the election, hundreds of thousands of Mousavi supporters marched in the street, declaring the results a fraud. At least 20 people were killed when police cracked down on the protesters, although some human rights groups say the death toll was much higher.

With files from The Associated Press