Iran will hold a landmark presidential election today amid warnings of violence if the vote is rigged to secure President Ahmadinejad’s return.

As the most tumultuous campaign in the Islamic Republic’s 30-year history drew to a close, Mir Hossein Mousavi, the incumbent’s strongest challenger, appealed to Ayatollah Ali Khamenei, the Iranian Supreme Leader, to ensure that the election was fair.

Those close to Mr Mousavi went further, claiming that Mr Ahmadinejad could win only by cheating — and that this could spark riots and chaos on the streets.

Saeed Laylaz, a political consultant, said he feared a “Tiananmen Square-style experience”, with the military moving in to crush any protests.

Such warnings were lent credence by Yadollah Javani, a leader of the hardline Republican Guard, who has vowed to suppress any attempt by Mr Mousavi’s reformist supporters to mount a “velvet revolution” after the election.

Officials expect a near-record turnout of the 46 million eligible voters after an extraordinary campaign marked by vast rallies, all-night revelry on the streets of Tehran and unprecedented infighting among the Iranian political elite.

The ultra-conservative Mr Ahmadinejad has again presented himself as a man of the people who has used his first term to challenge a corrupt Establishment at home, defy the American superpower and restore Iranian dig- nity. President Obama’s recent offer of dialogue was vindication for his efforts, he has claimed.

Although he has caused rampant inflation through his reckless spending of Iran’s record oil revenues, he has strong support among the pious and the rural poor, on whom he has lavished visits and largesse, from cheques to potatoes.

He also has the backing of Mr Khamenei, of the Republican Guard and its Basiji militia, and of a nationwide government machine that includes the state broadcaster.

Until a fortnight ago, Mr Ahmadinejad’s re-election looked all but certain, but urban middle-class Iranians began to rally behind Mr Mousavi, a former Prime Minister and relative moderate. He was a dull candidate, but he promised social liberalisation — including an end to the hated morality police — sound economic management and détente with the West. He also broke with tradition by allowing his much more forceful wife, Zahra Rahnavard, to campaign publicly with him.

Mr Mousavi is scarcely a reformist, but young Iranians and women have turned to him as their best chance of ousting a President who has sharply curtailed their personal freedoms. They have flooded on to the streets of Iran’s cities in huge numbers to demonstrate their support.

The Mousavi campaign encouraged the bandwagon by adopting the colour green, which sprouted everywhere, and through the prodigious use of text messages and e-mails. As a fortnight of frenzied political activity finally subsided yesterday, Mr Laylaz and Mohammad Atrianfar, a senior adviser to Mr Mousavi, confidently predicted that he would win 55 to 60 per cent of the vote.

Iranian elections are notoriously unpredictable, however. There are no reliable opinion polls, and the result will be influenced by three extraneous factors.

The first is turnout. In past elections many reform-minded Iranians have refused to participate, lest they legitimise the political system, but they are expected to vote in large numbers today to prevent Mr Ahmadinejad from winning a second term.

Second, the moderate vote will split between Mr Mousavi and Mehdi Karoubi, a former parliamentary Speaker, while a fourth candidate, Mohsen Rezai, a former commander of the Republican Guard, will lure some disaffected conservatives away from Mr Amadinejad.

Third, there is a danger of voterigging of the sort that is widely believed to have secured Mr Ahmadinejad, a rank outsider, the presidency in 2005. His men now control the Interior Ministry and the committee that oversees the elections. Independent experts believe that he could secure as many as four million extra votes through fraudulent ballot papers, miscounting and other measures.

If no candidate secures 50 per cent of today’s vote there will be a run-off next Friday between the top two — presumably Mr Ahmadinejad and Mr Mousavi. Most experts believe that the anyone-but-Ahmadinejad vote would coalesce behind his main rival and hand him victory.

The campaign has for the first time exposed deep rifts within the regime, with Mr Ahmadinejad and his three opponents hurling accusations of corruption, fraud and nepotism at each other in an unprecedented series of live television debates.

“This campaign shows the Islamic Republic of Iran is not a united entity any more,” said Mr Laylaz.

It has also laid bare the deep divisions in Iranian society between urban and rural, secular and devout, young and old.

Curiously, foreign policy was hardly an issue, despite Mr Ahmadinejad’s constant provocation of the West, Iran’s pursuit of nuclear weapons and Mr Obama’s overtures after the deep freeze of the Bush years.

It is Ayatollah Khamenei and not the President who controls Iran’s foreign and nuclear policies. However, diplomats hope for a change of tone, if not substance, if Mr Mousavi is elected and believe that could lead to a resumption of negotiations over Iran’s pursuit of nuclear power.

All four candidates have pledged to continue the nuclear programme — a source of great national pride that they insist is for energy generation only — but Mr Mousavi has promised to pursue a less abrasive foreign policy and is willing to meet Mr Obama.

During the campaign he accused Mr Ahmadinejad of reducing Iranian passports to the status of Somalia’s by denying the Holocaust and other antics.

“Our people have not given you the right to disgrace them,” he said.

Times Recommends

• Please help: maimed children of war want their lives back
• Cabal seizes diamond riches to buy power
• White House gatecrashers got to the president