TEHRAN – Iran's supreme leader will address the nation on Friday for the first time since a disputed election result triggered the biggest protests the Islamic Republic has seen.

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

Ayatollah Ali Khamenei has urged Iranians to unite behind hardline President Mahmoud Ahmadinejad but supporters of defeated candidate Mirhossein Mousavi have so far ignored the call, holding huge rallies in defiance of an official ban.

Khamenei's speech at Friday prayers in the Iranian capital follows a sixth day of protests by Mousavi supporters. On Thursday, tens of thousands, wearing black and carrying candles, marched to mourn those killed in earlier mass rallies.

The largest and most widespread demonstrations since the 1979 Islamic revolution have rocked the world's fifth biggest oil exporter, which is also caught up in a dispute with the West over its nuclear program.

Iranian state media has reported seven or eight people killed in protests since the election results were published on June 13. Scores of reformists have been arrested and authorities have cracked down on both foreign and domestic media.

Iranian Nobel Peace Prize winner Shirin Ebadi said about 500 people had been arrested in the last week, and called for their unconditional release. She said Iran should hold new elections under the supervision of the United Nations.

Mousavi, a moderate who advocates better ties with the West, has also called for the election to be annulled, saying pledges by the country's top legislative body, the Guardian Council, to recount some disputed ballot boxes did not go far enough.

The council has invited Mousavi and two other defeated candidates to talks on Saturday, and says it has begun careful examination of 646 complaints.

Objections include a shortage of ballot papers, pressure on voters to support a particular candidate, and the barring of candidates' representatives from polling stations.

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.

BLOODIED FACES

At Thursday's rally, protesters massed in a Tehran square, responding to Mousavi's call for people to gather in mosques or at peaceful rallies to show solidarity with the victims and their families.

They held photographs of those killed, some showing bloodied faces, apparently taken after they died.

Our martyred brothers we will take back your votes, read one placard. Why did you kill our brothers? said another.

Other banners told protesters to stay home on Friday, when Ahmadinejad supporters are expected to show their strength at Khamenei's Friday prayers, but to gather again the next day.

Mousavi supporters say he will be joined on Saturday by reformist former president Mohammad Khatami and another defeated candidate, liberal cleric Mehdi Karoubi.

Ahmadinejad has defended the legitimacy of the vote, telling a cabinet meeting on Wednesday that 25 million of 40 million voters had approved the way he was running the country.

The semi-official Fars news agency said two children of powerful cleric Akbar Hashemi Rafsanjani, who backs Mousavi and clashed with Ahmadinejad during campaigning, had been barred from leaving Iran.

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

Iran's Intelligence Ministry said it had uncovered a foreign-linked terrorist plot to plant bombs in mosques and other crowded places in Tehran during the election.

State broadcaster IRIB quoted a ministry statement as saying several terrorist groups had been discovered, adding they were linked to Iran's foreign enemies, including Israel.

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 but the overall result would not change because there isn't a millionth chance of doing any fraud.

(Editing by Jon Hemming)