Hundreds of thousands of protesters dressed in black and green flooded the streets of Tehran on Thursday in a somber, candlelit show of defiance and mourning for those killed in clashes after Iran's disputed presidential election.

The massive march — the fourth this week — sent a powerful message that opposition leader Mir Hossein Mousavi has the popular backing to sustain his unprecedented challenge to Iran's ruling clerics.

Even President Mahmoud Ahmadinejad, named the landslide winner in the June 12 election, appeared to take the growing opposition more seriously and backtracked on his dismissal of the protesters as "dust" and sore losers.

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The government tried to placate Mousavi and his supporters by inviting him and two other candidates who ran against Ahmadinejad to a meeting Saturday with Iran's main electoral authority, the Guardian Council. Abbasali Khadkhodaei, a spokesman for the council, said it received 646 complaints from the three candidates.

Mousavi accuses the government of widespread vote-rigging and demands a full recount or a new election, flouting the will of Iran's supreme leader, Ayatollah Ali Khamenei — a man endowed with virtually limitless powers under its constitution.

Many in the huge crowd walked silently and lit black candles as night fell. Others wore green wristbands or ribbons and carried flowers as they filed into Imam Khomeini Square, a large plaza in the heart of the capital named for the founder of the Islamic Revolution, witnesses said.

Mousavi, dressed in a black suit, was almost swallowed up by the throng as he addressed them briefly through a handheld loudspeaker. Press TV, an English-language version of Iranian state television designed for foreigners, said he called for calm and self-restraint from the crowd that the broadcaster estimated in the hundreds of thousands.

Foreign news organizations have been barred from reporting on Tehran's streets.

For the fifth straight night, Ahmadinejad opponents went to their rooftops in Tehran and cried out "Allahu akbar!" — "God is great!" The rooftop shouting is a deeply symbolic tactic that Mousavi borrowed from the Islamic Revolution and the idea that people power can challenge any system. The rooftop cries were how Khomeini asked Iran to show its unity against the shah 30 years ago.

Hundreds of thousands, including middle-class families and religious men and women, have flocked to Tehran's streets in recent days to declare their support for Mousavi. Similar, smaller protests have popped up in other cities in Iran.

Protesters have focused on the results of the balloting rather than challenging the Islamic system of government. But a shift in anger toward Iran's non-elected theocracy could result in a showdown over the foundation of Iran's system of rule.

"I don't think everyone wants to end the Islamic Republic because many people in Iran are very religious. So I think this current movement should keep Islam in it to maintain support. Unity is important," said a 29-year-old engineering graduate.

He, like the other witnesses the AP talked to, spoke on condition of anonymity because they feared reprisal.

The demonstrators marched silently until they reached the central square, where some chanted "Death to the dictator!" a witness said. Another said protesters also warned the government: "We will not get exhausted and we will come every day."

Television footage showed protesters making V-for-victory gestures and holding pictures of Mousavi and signs that say "Where's our Vote?"

The groundswell of support appears to have taken Iran's leaders — and even Mousavi supporters — by surprise.

This week's rallies openly defied orders from Khamenei, who has urged the people to pursue their allegations of election fraud within the limits of the cleric-led system.

Thursday's march was similar to one on Monday, when hundreds of thousands turned out in a huge procession that recalled the scale of protests during the 1979 Islamic Revolution which ended the monarchy. Seven demonstrators were shot and killed that day by pro-regime militia in the first confirmed deaths during the unrest.

The crowds in Tehran and elsewhere have been able to organize despite a government clampdown on the Internet and cell phones. 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. Other sites are slow to connect.

Text messaging, which is a primary source of spreading information in Tehran, has not been working since last week, and cell phone service in Tehran is frequently down.

The government initially tried to dismiss Mousavi's election allegations and supporter anger, but after four days of sustained protest, Ahmadinejad appeared to backtrack on his criticism and take the growing opposition more seriously.

"I was only addressing those who rioted, set fires and attack people. I said they are nothing," Ahmadinejad said in a previously taped video shown Thursday on state TV. "Every single Iranian is valuable. Government is a service to all."

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.

"I don't think the supreme leader was that upset about the idea of Mousavi being president. What he was upset about was the image of this green revolution and this wave of ordinary people having people power," said Patrick Clawson, deputy director at the Washington Institute for Near East Policy. "Certainly now the big concern is if you give into these people, that suggests these sorts of popular protests can succeed, and that's not good from Khamenei's perspective."

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.

Ahmadinejad appears similarly unable to draw the support of the country's highest-ranking clerics, whose rulings are followed by vast numbers of Iranians. Many congratulated Khamenei for holding the election but any mention of Ahmadinejad's victory was noticeably absent.

Mohsen Rezaei, a conservative who also ran for president, said there was pressure on some religious leaders to support the election results.

"Over the past three nights, some go to a nearby city and harshly ask why they do not take their stance (to support the results)," Rezaei said on state TV Thursday, without elaborating.

Some analysts suggest Mousavi, who was prime minister in the 1980s, is not looking to change the core foundations of Iran's Islamic regime.

"He is a leader of a revolution against a system of which he's been an integral part of," said Mehran Kamrawa, director of the Center for International and Regional Studies at Georgetown University in Qatar.

There have been widespread accusations of nighttime attacks on Mousavi supporters by pro-government militia, and protesters attacked a militia building after one rally. But both sides have been restrained, with uniformed police and other security forces standing by as protesters march calmly through the streets.

The Iranian government directly accused the United States of meddling in the deepening crisis. A statement by state-run Press TV blamed Washington for "intolerable" interference. The report, on Press TV, cited no evidence.

On Thursday, state TV said four people had been arrested in Iran for allegedly plotting to set off bombs on election day. It aired what it described as a confession by one of the arrested, who said he has been working with Americans.

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The Associated Press contributed to this report.