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Tens of thousands of people have joined a rally in central Tehran to celebrate the re-election of Iran's President Mahmoud Ahmadinejad.

Crowds thronged the main thoroughfare, Vali Asr street, waving Iranian flags and chanting in jubilation.

The president's closest opponent in the election, Mir Hossein Mousavi, has lodged an official appeal against the result amid continuing angry protests.

Mr Ahmadinejad denied any vote-fixing, saying the result was "very accurate".

Streets sealed off

He told the rally that elections in the Islamic republic had never been healthier and that the people alone had decided the outcome.

"Some people want democracy only for their own sake," he said, referring to his critics both inside and outside of Iran.

"Some want elections, freedom, a sound election. They recognise it only as long as the result favours them."

He said Iranians were united, but with 40 million people taking part it in the election it was natural for some to be disappointed.

Clashes between the police and anti-Ahmadinejad protesters continued on Sunday, with several parts of the city sealed off by the authorities to prevent the unrest spreading.

Earlier, Mr Ahmadinejad told reporters that the debate over Iran's nuclear programme "belongs to the past", and that Iran had "embraced" the idea of an international effort to eliminate nuclear weapons.

US doubt

Global reaction to the election has been muted, but US Vice-President Joe Biden told broadcaster NBC he had doubts about the result.

Violent clashes have continued for most of the weekend

"There's an awful lot of questions about how this election was run, but we'll see, we're just waiting to see, we don't have a enough facts to make a firm judgement," he said.

Mr Ahmadinejad's closest rival in the election campaign, Mir Hossein Mousavi, announced on Sunday he had lodged an official appeal appeal against the result to Iran's Guardian Council .

"I urge you, Iranian nation, to continue your nationwide protests in a peaceful and legal way," he said in a statement.

Mr Mousavi has called several times for his supporters to avoid violence, but angry protesters have been setting light to vehicles and throwing stones in Tehran.

Reuters reported that police charged a 2,000-strong group of students who were protesting at the University of Tehran.

Leader's endorsement

Dozens of activists have been arrested, with reports saying those detained were members of pro-reformist political parties which had backed Mr Mousavi during the election campaign.

Those held were said to include Mohammad Reza Khatami, brother of former President Mohammad Khatami.

Many of them are reported to have been freed since.

The state news agency, Irna, said those arrested were involved in orchestrating Saturday's protests in Tehran.

The controversy flared after a high turnout on Friday - estimated at 85% - led Mr Mousavi's supporters to expect a close result.

But official results gave Mr Ahmadinejad a landslide victory. His final share of the vote was almost 63%.

The result was quickly endorsed by Ayatollah Khamenei, Iran's supreme leader.

Our correspondent John Simpson, in Tehran, says the truth about the election may never be known.

But many observers in Iran feel that the final result did not reflect the extraordinary numbers of people who turned out to vote, our correspondent adds.