TEHRAN -- 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.

Defeated Iranian presidential candidate Mirhossein Mousavi speaks to supporters at a rally in Tehran June 18, 2009. [Agencies]

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.

The government invited 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.