Iran's supreme leader will conduct a prayer congregation Friday at Tehran University, days after violent clashes there over the country's hotly-disputed presidential election, Tehran's official news service reported.

Ayatollah Ali Khamenei will lead Friday prayers on the campus, IRNA reported, amid mass protests against the election results. Backers of opposition leader Mir Hossein Mousavi have poured into the streets of Tehran Thursday in what he has called "a day of mourning" of those killed in post-election violence.

Many in the massive crowd wore green wristbands and carried flowers in mourning as they filed into Imam Khomenei Square, a large plaza in the heart of the capital named for the founder of the Islamic Revolution, witnesses said.

Meanwhile, state radio reported that Iran's Guardian Council, an unelected body of 12 clerics and Islamic law experts close to Khamenei, will meet Saturday with election candidates to discuss complaints about the voting, according to Reuters.

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Demonstrators marched silently until they arrived at the square, where some chanted "Death to the Dictator!" and "Where are our votes!"

Mousavi, who claims the election was rigged and he was the true winner, alleges that the Guardian Council is not neutral and has already indicated it supports Ahmadinejad. He wants an independent investigation.

Violent clashes erupted after the election commission announced Ahmadinejad had won in a landslide. Seven demonstrators were shot Monday by pro-regime militia in the first confirmed deaths during the unrest.

Thursday's protest would be the fourth straight day of major marches in Tehran. On Monday, hundreds of thousands turned out in a huge procession that recalled the scale of protests during the 1979 Islamic Revolution. Mousavi's Web site said he may join the rally on Thursday in downtown Tehran.

Ahmadinejad released a largely conciliatory recorded statement on state TV, distancing himself from his past criticism of protesters, whom he compared to "dust" and sore losers after a soccer match.

"I only addressed those who rioted, set fires and attacked people," the statement said. "Every single Iranian is valuable. The government is at everyone's service. We like everyone."

On Wednesday, thousands marched silently down a main street in the capital, holding posters of Mousavi and flashing the V-for-victory sign in the air, amateur video showed.

The street protests have presented one of the gravest threats to Iran's complex blend of democracy and religious authority since the system emerged out of the 1979 Islamic revolution.

Any serious shift of the protest anger toward Iran's non-elected theocracy would sharply change the stakes. Instead of a clash over the June 12 election results, it would become a showdown over the core premise of Iran's system of rule — the almost unlimited authority of the clerics at the top.

The Iranian government accused the United States on Wednesday of meddling in the deepening crisis. State media blamed Washington for "intolerable" interference.

"Despite wide coverage of unrest, foreign media have not been able to provide any evidence on a single violation in the election process," state radio said Thursday.

Authorities have rounded up perceived dissidents and tried to further muzzle Web sites and other networks used by Mousavi's backers to share information and send out details of the crisis after foreign journalists were banned from reporting in the streets.

President Obama said he shared the world's "deep concerns" but it was "not productive, given the history of U.S.-Iranian relations, to be seen as meddling." The two countries severed diplomatic relations after the 1979 Islamic Revolution in the bloody showdown over allegations of vote-rigging and fraud.

A crackdown on dissent continued, with more arrests of opposition figures reported, and the country's most powerful military force — the Revolutionary Guard — saying that Iranian Web sites and bloggers must remove materials that "create tension" or face legal action.

The government has blocked certain Web sites, such as BBC Farsi, Facebook, Twitter and several pro-Mousavi sites that are vital conduits for Iranians to tell the world about protests and violence. Many other sites, including G-mail and Yahoo, were unusually slow and rarely connect.

Mousavi has condemned the blocking of Web sites, saying the government did not tolerate the voice of the opposition.

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On Thursday, Mousavi's Web site said that both Mousavi and former reformist President Mohammad Khatami sent a joint letter to Iran's head of judiciary, Ayatollah Mahmoud Hashemi Shahroudi, asking him to take measures to stop violence against protesters by police and help to release detained demonstrators.

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The Associated Press contributed to this report.