TEHRAN, Iran – In a sign of endurance for Iran's protest movement, demonstrators clashed with police Friday as one of the nation's most powerful clerics challenged the supreme leader during Muslim prayers, saying country was in crisis in the wake of a disputed election.

The turnout of tens of thousands of worshippers for former President Akbar Hashemi Rafsanjani's sermon at Tehran University and the battles with police outside represented the biggest opposition show of strength in weeks. Protesters faced fierce government suppression and hundreds were arrested following the disputed June 12 presidential election.

Outside the university, protests grew from several hundred before the sermon to thousands afterward as worshippers joined in, chanting, "death to the dictator," a reference to President Mahmoud Ahmadinejad.

Protesters were confronted by riot police and a menacing line of pro-government Basiji militiamen on motorcycles, who charged with batons. Plainclothes Basijis fired volleys of tear gas, and young protesters with green bandanas over their faces kicked the canisters across the pavement. Some set a bonfire in the street and waved their hands in victory signs. Dozens were arrested, taken away in trucks, witnesses said.

Protests, which flared following the election, had been stifled in recent weeks. The sometimes tearful sermon by Rafsanjani could be a significant boost to the movement's staying power. It was an open challenge to Supreme Leader Ayatollah Ali Khamenei, aired live on nationwide radio from one of the country's most potent political stages. By openly showing the divisions in the leadership, it punched a hole in efforts by Khamenei and hard-line clerics to end the controversy over Ahmadinejad's re-election.

Worshippers chanted "azadi, azadi," Persian for "freedom," during Rafsanjani's sermon, his first since the election. Opposition leader Mir Hossein Mousavi, who claims to have won the election, sat among the worshippers, attending the country's main prayer service for the first time since the turmoil began.

Many of those gathered wore headbands or wristbands in his campaign color green, or had green prayer rugs, crowding the former soccer field where prayers are held and spilling into nearby streets.

Rafsanjani denounced the government crackdown on protests and called for the release the hundreds detained.

He reprimanded the clerical leadership for not listening to people's complaints over the election, which was declared a victory for Ahmadinejad despite opposition accusations of fraud.

"There is a large portion of the wise people who say they have doubts (about the election). We need to take action to remove this doubt," he said. "The trust that brought the people to vote in such large numbers is not there anymore. We need to return this trust."

Rafsanjani avoided directly mentioning Khamenei or outright calling the vote fraudulent. He couched his sermon in calls for unity in support of Iran's Islamic Republic, but it was clear he blamed the leadership for the loss of unity.

The cleric got tears in his eyes as he spoke of how Islam's Prophet Muhammad "respected the rights" of his people. He said the founder of Iran's Islamic Republic, Ayatollah Ruhollah Khomeini, "would always say that if the system is not backed by the people, nothing would stand."

For Iranians listening across the country, the weekly Friday sermon in Tehran is the voice of the leadership and a symbol of its backing by God.

After hundreds of thousands joined protests against the election results in the days following the vote, the supreme leader used the podium to declare Ahmadinejad's victory valid and order a stop to unrest. The crackdown was launched soon after. In the weeks that followed, hard-line clerics have used the sermon to depict the protesters as tools of foreign enemies and tell worshippers to follow Khamenei.

Rafsanjani's sermon lay bare to the broader public that the dispute was internal and even Iran's ruling clerics are split. He directly referred to the divisions, saying the revered topmost theologians of Shia Islam, who have millions of followers, were not happy with the government.

Rafsanjani heads two powerful clerical bodies that oversee the government and parliament, the Expediency Council and the Experts Council. He is bitter rival of Ahmadinejad and is considered Mousavi's top supporter within Iran's clerical leadership.

A mercurial and savvy politician, Rafsanjani positioned himself as a leader emerging to resolve the unrest, saying he hoped his words would be a start to "help us pass safely through a problem that can unfortunately be called a crisis."

He specifically criticized his top rivals within the clerical leadership — the Guardians Council, a body dominated by hard-liners. The council oversaw the election, then held a partial recount that upheld Ahmadinejad's win but was dismissed by the opposition.

Rafsanjani said the Guardians Council missed an "opportunity to unite the people and regain their trust."

Inside the prayers, worshippers traded competing chants with some hard-liners in the congregation. When the hard-liners chanted "death to America," Mousavi supporters countered with "death to Russia" and "death to China," a reference to Ahmadinejad's alliance with both countries.

The Iranian government has accused the U.S. and other Western countries of inciting the massive street protests and interfering with the election. On Thursday, Ahmadinejad demanded and apology from the U.S. as a step toward dialogue between the two countries.

"They tried to interfere in our elections. They talked nonsense. They were rude. They fomented aggression against people's wealth and property," Ahmadinejad told a crowd of thousands in the northwestern city of Mashhad.

The U.S. has denied the government's allegations.

More than 500 remain in prison following the government's crackdown and at least 20 were killed. In the past three weeks, the opposition held only one other significant protest before Friday's.

The scene outside the university on Friday was tumultuous. Before the sermon, police fired tear gas at hundreds of Mousavi backers trying to enter.

When Mahdi Karroubi, another pro-reform candidate in the June election, headed for the prayers, plainclothes Basijis attacked him, shouting "death to the opponent of Velayat-e-Faqih," or supreme leader, witnesses said.

Also arrested was a prominent women's rights activist, Shadi Sadr, who was beaten by militiamen, pushed into a car and driven away to an unknown location, Mousavi's Web site said.

Protests died down by nightfall. After sunset, Iranians could be heard shouting from rooftops, "God is great" and "death to the dictator" — a show of opposition support that has been held every night since the election, but appeared louder and more widespread Friday night.