Opposition supporters take to the streets of Tehran once again
Supporters of Iran's defeated presidential candidate Mir Hossein Mousavi have held another big rally in northern Tehran.
Hours before, thousands of President Mahmoud Ahmadinejad's supporters staged their own protest in the city centre.
The opposition rally went ahead despite an official offer of a partial recount of the poll, which returned Mr Ahmadinejad to power.
Tough new restrictions have been imposed on foreign media reporting.
The curbs came amid apparent surprise and concern among the authorities at the scale of popular defiance over Friday's official election results, correspondents say.
Jon Leyne Reporting from Tehran
In a way we've reached a bit of a deadlock. The government can't work out how to deal with the unprecedented demonstrations. But the opposition is disorganised, it has no co-ordination, no strategy.
And the demonstrators, despite the radical nature of what they are doing, don't fully perceive the way in which they are threatening the foundations of Islamic Republic. This has gone way beyond disputed elections.
I don't think the offer of a recount will have much effect. They haven't said who would do the recount, what about irregularities on election day? Nothing less than a full re-run of the election would satisfy the opposition. I can't see how government would offer that. The Supreme Leader has staked so much on the results of this election, it would be a massive u-turn.
President Ahmadinejad was declared the easy victor of the presidential poll on Saturday, with results giving him 63% of votes against 34% for Mr Mousavi.
But the opposition alleged widespread irregularities.
The powerful Guardian Council says it is ready to recount some votes from the poll. A spokesman for the Council, Abbas Ali Kadkhoda'i, told the state broadcaster Irin that the council had met representatives of the presidential candidates and would look into their allegations.
But opposition candidates have demanded a full re-run of the election.
On Tuesday evening, Ayatollah Khamenei - who has backed the Guardian Council's recount - called for "tolerance" and "patience" during a TV address on Tuesday evening.
But he also referred to Mr Ahmadinejad as the country's elected president - apparently prejudging the recount, said correspondents.
Anger at the official result saw hundreds of thousands of Mr Mousavi's supporters take to the streets on Monday - in a rally the size of which correspondents said not been seen in Tehran since the 1979 revolution.
A witness told the BBC that Tuesday's rally was even bigger than Monday's - though this cannot be independently confirmed - and the state Press TV also described it as large.
Witnesses described demonstrators walking in near silence towards state TV headquarters - apparently anxious not to be depicted as hooligans by authorities.
Thousands of Ahmadinejad supporters also rallied in Tehran
Thousands of supporters of President Ahmadinejad staged their own rally in Vali Asr Square in central Tehran - some bussed in from the provinces, correspondents say.
The latest opposition rally comes despite a Mousavi spokesman urging supporters not to take part in another demonstration on Tuesday, amid fears of new violence.
Iran's police chief, Gen Ahmadi Moqaddam, has warned action will be taken against any unauthorised protest, and "will quell any unrest".
Hospital officials say eight people died in violence which erupted at the end of Monday's rally, which authorities blamed on "thugs".
Since then, the authorities have imposed tough new restrictions on foreign journalists operating in Tehran - the most sweeping restrictions our correspondent in Tehran, Jon Leyne, says he has ever faced.
They must now obtain explicit permission before leaving the office to cover any story.
Journalists have also been banned from attending or reporting on any "unauthorised" demonstration - and it is unclear which if any of the protests are formally authorised.
Some telephone, SMS and internet services have also been restricted, prompting some protesters to turn to the internet messaging service Twitter to communicate.
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
The importance of such new means of communication was highlighted by a US official on Tuesday.
The official said the state department contacted Twitter over the weekend to urge it to delay a planned upgrade that could have cut daytime service to Iranians.
Dozens of people have been arrested since the protests began - including Mohammad Ali Abtahi, a close aide of ex-President Mohammad Khatami, and journalist and academic Ahmad Zeidabadi.
On Tuesday, lawyer Abdolfatah Soltani of the Human Rights Defenders' Centre was arrested, a colleague told AFP news agency. The centre is led by Iranian Nobel peace prizewinner Shirin Ebadi.
Unrest has also been reported in other Iranian cities including Mashhad, Isfahan and Shiraz.
In Isfahan, a demonstrator, Arash, told the BBC that the religious police initially tried to quash a protest using tear gas and throwing stones, but that they had retreated as the size of the crowd grew.
"Then people headed towards a building where the religious police are based, because they were angry that the religious police had attacked them with stones.
"So people set their bikes on fire. They started shouting slogans like 'death to the dictator', and they were inviting the army and the police to join the real people," he told the BBC.
It's young people themselves who are organising these protests... It's just word of mouth and a feeling of anger that gets people out on the streets
Arash added that it was a sense of disillusionment with Iranian politics that was driving the protests.
"It's young people themselves who are organising these protests... It's just word of mouth and a feeling of anger that gets people out on the streets."
In Washington, President Barack Obama again expressed "deep concern" at events in Iran, but said it would not be helpful if the US was seen to be "meddling".
Earlier, EU foreign ministers expressed "serious concern" and called for an inquiry into the conduct of the election.
But the Iranian authorities have bristled over the criticism. On Tuesday the Iranian foreign ministry summoned the Czech charge d'affaires in Tehran to complain over the EU's "rude and interfering" remarks.
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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