Iran's leading moderate party said on Wednesday last month's presidential election was a "coup d'etat" and the official result was "unacceptable", in a statement posted on its website.

"The election was the result of a year long coup d'etat ... that harmed the establishment's legitimacy inside and outside Iran ... We openly announce that the result is unacceptable," said the statement of Islamic Iran's Participation Front.

It also urged people, political parties and groups to avoid any move that could be used as a pretext by "violence-seekers" to pave the ground for "dictatorship".

"We are asking the authorities ... that now after establishing what they wanted to, to release all those jailed, innocent people and avoid creating fake cases for them," the statement said.

The party was formed by reformers close to former president Mohammad Khatami, including his brother, after his landslide election win in 1997.

Some members of the party have been arrested following the dispute over the June 12 presidential election, which defeated moderate candidates say was rigged in favour of hardline President Mahmoud Ahmadinejad. The authorities say it was the "healthiest" election since Iran's 1979 Islamic revolution.