Iranian state TV announced that seven people had been killed
Iranian state radio says seven people were killed during Monday's protests in Tehran over the re-election of President Mahmoud Ahmadinejad.
The reports said the deaths came after "thugs" attacked a military post.
The country's powerful Guardian Council has described the results of Friday's disputed poll as "provisional".
Opposition candidate Mir Hossein Mousavi wants them annulled, alleging widespread fraud. Rival marches are planned at the same site on Tuesday.
The demonstrations called by supporters of both President Ahmadinejad and Mr Mousavi are due to take place in Vali Asr Square in central Tehran.
Monday's protest involved hundreds of thousands of people and was one of the largest since the Iranian revolution 30 years ago.
CHANGE OF MOOD
Jon Leyne Reporting from Tehran
I certainly get the impression from the authorities that the wind is really changing here.
The Guardian Council has now said it was only a provisional result and has called in all three losing candidates to speak to them this morning.
Bizarrely, all the communications seem to be working - suddenly international phone calls are working. The BBC satellite is not being blocked.
At the same time, Mr Ahmadinejad has gone off to a regional summit in Russia, apparently oblivious to everything.
He didn't even mention the trouble when he spoke to reporters at the airport just before he left.
The radio report said the attack occurred at the end of the "illegal" rally as people were heading home "peacefully".
"Several thugs wanted to attack a military post and vandalise public property in the vicinity of Azadi Square," the radio said referring to the site of the protest.
"Unfortunately seven people were killed and several others wounded in the incident."
The BBC's Jon Leyne in Tehran says that in light of what he saw of the vast and largely peaceful protests this seems an unlikely version of events.
Dozens of opposition activists have been arrested since the protests began.
A number of senior reformist politicians, including former Vice-President Mohammad Ali Abtahi and Mousavi ally Saeed Hajarian, were detained overnight, reports said.
Unrest has been reported in other parts of Iran. One of Mr Mousavi's websites said a student had died on Monday in clashes with hardliners in the southern city of Shiraz.
Our correspondent adds that the authorities appear to be weakening in their support for President Ahmadinejad.
Iran's most powerful body, currently controlled by conservatives
Includes six theologians picked by Supreme Leader and six jurists approved by parliament
Half the members change every three years
Approves bills passed by parliament and can veto them if deemed inconsistent with the constitution or Islamic law
The council can also bar candidates from standing in elections
Parliamentary Speaker Ali Larijani condemned the response to the protests. Iranian media quoted him as saying: "The interior minister is responsible in this regard."
The country's supreme leader, Ayatollah Ali Khamenei, has ordered an inquiry into the allegations of vote-rigging.
Mr Mousavi and two other moderate candidates contesting the results are due to meet members of the Guardian Council - a powerful clerical group dominated by conservatives.
On Monday a spokesman for the council said that the results had not yet been finalised, and that they would only be final once the council has endorsed them.
The handling of the protests by the authorities has drawn international criticism.
EU foreign ministers expressed "serious concern" and called for an inquiry into the conduct of the election.
Supporters of Mir Hossein Mousavi defied a ban on protest rallies
In his first comments on the situation, US President Barack Obama said he was "deeply troubled" by the violence in Iran.
He said he would continue pursuing tough dialogue with Iran.
"I think that the democratic process, free speech, the ability for folks to peacefully dissent, all those are universal values and need to be respected."
But the BBC's Jonathan Beale, in Washington, says the president studiously avoided any comment on the allegations of vote fraud.
"We respect Iranian sovereignty and want to avoid the United States being the issue inside of Iran," Mr Obama said.
Meanwhile, President Ahmadinejad arrived in Russia on Tuesday. He told a regional summit that the "age of empires" had ended, but made no mention of the protests.
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