TEHRAN- Tens of thousands of Mirhossein Mousavi's backers, wearing black and carrying candles, rallied in Tehran on Thursday to mourn those killed in mass protests against an election the defeated candidate says was rigged.

(Editors' note: Reuters and other foreign media are subject to Iranian restrictions on their ability to report, film or take pictures in Tehran.)

Days of public fury over the disputed election led Iran's top legislative body, the Guardian Council, to invite Mousavi and the two other candidates beaten by hardline President Mahmoud Ahmadinejad to discuss their complaints on Saturday.

The election has provoked Iran's worst unrest since the 1979 Islamic revolution. Bloodshed, protests, arrests and a media crackdown have rocked the world's fifth biggest oil exporter, embroiled in a dispute with the West over its nuclear program.

Witnesses said people packed Imam Khomeini Square in central Tehran on Thursday, a day after Mousavi called on his supporters to gather in mosques or at peaceful rallies.

He told them to wear the color of mourning -- black as opposed to the green of his election campaign -- in solidarity with families of those wounded or killed in the protests.

Where are our brothers? read one banner in the crowd. Why did you kill our brothers? read another.

Iran's English-language state television has reported eight people killed in protests since election results were published. Scores of reformists have been arrested across the country.

Security agents detained opposition politician Ebrahim Yazdi while he was in hospital, an ally of his said on Thursday. Yazdi heads the banned Freedom Movement and was foreign minister in Iran's first government after the revolution.

A spokesman for the Guardian Council said it had begun careful examination of 646 complaints submitted after the June 12 vote. Complaints include shortage of ballot papers, pressure on voters to support a particular candidate, and the barring of candidates' representatives from polling stations.

Ahmadinejad was declared winner with nearly 63 percent of the vote against 34 percent for his closest rival, Mousavi, a moderate politician who wants better ties with the West.

Mousavi wants the vote annulled and held again. The council has said it is ready only to recount disputed ballot boxes.

Council spokesman Abbasali Kadkhodai said Mousavi and fellow-candidates Mehdi Karoubi and Mohsen Rezaie could raise their problems at an extraordinary council meeting on Saturday.

CHALLENGE TO WESTERN DEMOCRACY

Ahmadinejad defended the legitimacy of the vote, telling a cabinet meeting on Wednesday that it had posed a great challenge to the West's democracy, Mehr news agency reported.

The ideals of the Islamic Revolution were the winners of the election, Ahmadinejad said, adding that 25 million of 40 million voters had approved the way he was running the country.

The authorities reject charges that they rigged the vote. Scores of thousands of Iranians have braved riot police and religious militia to show their anger on the streets, ignoring Supreme Leader Ayatollah Ali Khamenei's call for national unity.

The friendly atmosphere that existed prior to the election should not turn into an atmosphere of confrontation and enmity afterwards, since both groups of voters believe in the Islamic system, Khamenei was quoted as saying in Kayhan newspaper.

The supreme leader is due to lead Friday prayers, when Ahmadinejad supporters are expected to show their strength.

The semi-official Fars news agency said two children of conservative former president Akbar Hashemi Rafsanjani, who backs Mousavi, had been barred from leaving Iran.

Rafsanjani's daughter Faezeh addressed Mousavi supporters on Tuesday. Hardline students called for her and her brother, Mehdi, to be arrested.

Hamid Najafi, editor-in-chief of Kayhan International, an English-language conservative Iranian daily, said the Guardian Council investigation of the vote would calm unrest.

I think that will be enough to pacify the opponents, he told Reuters. After the investigation the Guardian Council will issue an official statement. That will be the final act.

He said the overall election result would not change because there isn't a millionth chance of doing any fraud and Ahmadinejad's victory was very obvious. Khamenei's Friday sermon would be very effective for all these people who have been misled, he added. Of course everything will calm down.

Iran has denounced foreign criticism of the election, although U.S. President Barack Obama's administration has muted its comments to keep the door open for possible dialogue.

(Additional reporting by Dominic Evans and Hossein Jaseb in Tehran; editing by Philippa Fletcher)