Supporters of opposition leader Mir Hossein Mousavi listen to his speech at a demonstration Thursday in Tehran.

Iranian opposition leader Mir Hossein Mousavi, center right, addresses supporters Thursday at a demonstration in Tehran, Iran, in this image made available from Mousavi’s election campaign media operation Ghalam News. Tens of thousands of black-clad protesters filled the streets of Tehran again Thursday, joining opposition leader Mir Hossein Mousavi to mourn demonstrators killed in clashes over Iran’s disputed election.

Tehran, Iran ? 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.

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.

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.