President Mahmoud Ahmadinejad has been sworn in for a second term
Supporters of the Iranian opposition leader, Mir Hossein Mousavi, have taken to the streets of Tehran shouting "Death to the dictator", reports say.
Witnesses told the Reuters news agency the protesters were at Vanak Square, where riot police were deployed.
Others were repeatedly hooting their car horns on nearby roads, they said.
On Wednesday, clashes broke out between police and opposition supporters after President Mahmoud Ahmadinejad was sworn in for a second term in office.
The opposition disputes the official result of the presidential election on 12 June, which gave Mr Ahmadinejad an overwhelming victory.
After the poll, thousands of people took to the streets in the largest mass demonstrations in Iran since the 1979 Islamic Revolution.
Nearly eight weeks after the election, hundreds of supporters of Mr Mousavi, who came second in June, again converged on Vanak Square in northern Tehran on Thursday for an anti-government demonstration.
"Hundreds of people are in Vanak Square, chanting 'Death to the dictator'. Others are also honking car horns," one witness said, reports Reuters.
"Hundreds of riot police are there as well. They are telling protesters to leave the area or face being arrested."
Mr Mousavi and the other defeated reformist candidate, Mehdi Karoubi, have said Mr Ahmadinejad's government will be illegitimate.
On his website on Wednesday, Mr Karoubi criticised the authorities for "suppressing street protests" and promised to continue the "fight".
Mr Ahmadinejad has received the backing of Supreme Leader Ayatollah Ali Khamenei, who says he was the rightful winner and that Iranians had "voted in favour of a fight against arrogance, to confront destitution and spread justice".
After taking his oath of office, Mr Ahmadinejad defended the official results and criticised Western powers' response to the disputed poll.
"[Foreign governments] just want democracy at the service of their own interests - they do not respect the rights of other nations," he said. "They see themselves as the yardstick of democracy - our people oppose this, that's what our people are resisting," he said.
Several states which have criticised the elections chose not to send customary letters of congratulations to Mr Ahmadinejad to mark his reappointment, including the US, Germany, France and the UK.
"Nobody in Iran is waiting for anyone's congratulations," Mr Ahmadinejad said.
Outside, hundreds of demonstrators gathered. There were reports of clashes with riot police and at least one arrest.
More than 100 people, including some members of the opposition, went on trial in on Saturday for their alleged involvement in the post-election violence.
Foreign media, including the BBC, have been restricted in their coverage of Iran since the election protests turned into confrontations with the authorities in which at least 30 people died.
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