Iran will emulate the “Great Satan” this week with six live, US-style debates between its presidential candidates for the first time in the 30-year history of the Islamic Republic.

The 90-minute encounters, which start tonight, come as independent observers say that Mir Hossein Mousavi, President Ahmadinejad’s strongest rival, is gaining momentum with 11 days left before polling day.

There are no reliable opinion polls in Iran but Mr Mousavi, a former Prime Minister, is urging his predominantly young and urban supporters to display green in their clothing or on their cars. Large numbers of people are doing so.

“The tide has begun to turn against Ahmadinejad in favour of the reformist camps,” said Zhand Shakibi, a lecturer in comparative government at the London School of Economics, who is in Tehran.

Anoush Ehteshami, a professor of international relations at Durham University, said that Mr Ahmadinejad would probably fail to secure the 50 per cent of the vote that he needs on June 12 to avoid a run-off.

In the one-to-one confrontations, Mr Ahmadinejad will face Mr Mousavi tomorrow and Mehdi Karoubi, the former parliamentary Speaker, who is his other reformist challenger, on Saturday.

The President’s challengers will attack his economic mismanagement, abrasive foreign policy and social repression. Mr Ahmadinejad says that he will retaliate by exposing corruption in the governments of his reformist predecessors.

Why Ayatollah Ali Khamenei, the Supreme Leader of Iran, has approved the debates is unclear. The regime is eager to boost turnout to legitimise Iran’s unique form of government — part theocracy, part democracy.

Election day is “one of the Iranian nation’s big tests before the eyes of the enemies,” Mr Khamenei said recently.

A large turnout is likely to would favour the reformist candidates whose supporters are hardermore difficult to energise than Mr Ahmadinejad’s hardline devotees.

The debates will bring unpredictability into an election where Mr Ahmadinejad — Mr Khamenei’s favoured candidate — is the clear frontrunner, and challengers to the President have struggled to secure equal airtime.

An increase in turnout is expected to favour the reformist candidates.

“The two reformist candidates will benefit since the debates will give them the unique opportunity to criticise and attack nationally on television the performance of the Ahmadinejad Government,” Dr Shakibi said.

The regime may be calculating that the populist President will trounce his opponents. Mr Ahmadinejad has a huge following among the pious and rural poor.

He will play the nationalist card by arguing will argue that he has raised Iran’s status in the world by defying the West and continuing his country’s the nuclear programme.

Mr Mousavi and Mr Karoubi will accuse Mr Ahmadinejadhim of squandering record oil revenues through reckless spending programmes, leaving Iran isolated in the world, and curtailing personal freedoms. They champion greater rights for women and their wives have broken with tradition by campaigning actively.

Mr Ahmadinejad said that he will release damaging internal documents from past governments including, presumably, that over which Mr Mousavi presided as Prime Minister from 1981 to 1989.

“God willing, I will announce the list of known economic swindlers in upcoming television debates — an action that I believe would be in the best interests of the country and would alarm people about their actions,” Mr Ahmadinejad said.

The election is taking place against a background of mounting violence between Shias and Sunnis, which has included arson and bomb attacks. Five people were killed in an arson attack on a bank in the southeastern city of Zahedan yesterday.

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