In 50 Iranian cities the number of votes cast in this month presidential election exceeded the number of eligible voters, the state's election watchdog admitted today.

The surprising admission by the Guardian Council was, however, designed to undermine the claims of the defeated candidates that the vote was rigged.

Mir Hossein Mousavi, President Mahmoud Ahmadinejad's main rival in the hotly-disputed election, and the other two losing candidates have claimed that the vote exceeded eligible voters in as many as 170 districts.

Abbasali Kadkhodai, a spokesman for the council of senior clerics, told the state television channel IRIB: "Our investigation shows that the number of districts they announced is not correct. Based on our preliminary report, 50 districts face this issue."

Mr Kadkhodaei also argued that voter turnouts of more than 100 per cent were not unusual because Iranians can cast their ballots where they want. Although it is summer in Iran and some of the cities in question are in desert areas, he suggested some voters might have gone to them on holiday.

He also made clear that Mr Ahmadinejad's victory would not be affected even if the results in the 50 disputed districts were reversed as they account for three million votes and the official results show Mr Ahmadinejad won by a margin of 11 million.

Mr Mousavi and the other two candidates, Mehdi Karoubi and Mohsen Rezai, have all complained to the Guardian Council of electoral fraud and have reported 646 alleged irregularities.

The council has agreed to investigate those complaints but it is chaired by a close ally of Mr Ahmadinejad's and dominated by conservatives. The defeated candidates do not consider it impartial - especially after Ayatollah Ali Khamenei, the Supreme Leader, made clear in a speech on Friday that Mr Ahmadienjad's victory would stand.

Mr Mousavi and Mr Karoubi are demanding a fresh election. They refused an invitation to appear before the council to present their complaints on Saturday and Mr Mousavi has rejected its offer to recount a random ten per cent of the votes.

The regime also announced today that 457 people were arrested during Saturday's running battles on the streets of Tehran. It said they had "caused insecurity, disrupted public order and clashed with police".

At least 13 civilians died in those confrontations between pro-Mousavi demonstrators and the security forces. The prosecutor general's office in Tehran claimed that it was "unknown vandals", not the police, who opened fire on civilians.

The protests have prompted unprecedented traffic on the micro-blogging website Twitter.com and on other social networking sites, including Facebook and YouTube, as details of planned demonstrations are shared around.

Many of today's postings were focused on Neda Soltani, the 26-year-old woman allegedly shot by a Basiji militiaman on Saturday evening who has become a posthumous poster girl for the protests after her death was captured on video. YouTube users posted picture galleries and even a song in tribute to her today.