Tensions flared in Tehran last night as thousands of protesters marched to the state television centre, enraged by the discovery that President Ahmadinejad was being given far more airtime than his opponents.

The demonstration came as a leading conservative accused reformists of fomenting a “velvet revolution”. Yadollah Javani, a leader of the hardline Revolutionary Guards, said that reformists were going to claim vote-rigging if their candidate, Mir Hossein Mousavi, loses in tomorrow’s presidential elections. He vowed to crush any post-election violence.

As the accusations flew, Hojatoleslam Ali Akbar Hashemi Rafsanjani, a former President and powerful cleric, wrote an open letter to Ayatollah Ali Khamenei, Iran’s Supreme Leader, complaining about allegations of corruption that Mr Ahmadinejad had levelled against his family in a televised debate last week. He warned that Mr Khamenei’s failure to act against those “lies” could spark social unrest.

“Even if I keep quiet, part of the people, groups and parties will undoubtedly not bear this situation any more and set off social upheavals,” said Hojatoleslam Rafsanjani, who is backing Mr Mousavi.

In another remarkable intervention, about 50 clerics from Qom, Iran’s religious nerve-centre, also rebuked Mr Ahmadinejad, stating in an open letter that “accusing people in a session when they are not present is contrary to sharia.” Such conduct would “spread disappointment among the people and make our enemies happy”.

These developments expose deep splits within a ruling establishment that never normally airs its disagreements in public and come on the eve of a landmark election in which Mr Mousavi, a relative moderate, could become the first challenger to defeat an incumbent president in the Islamic Republic’s 30-year history.

There are no reliable opinion polls in Iran, but Mr Mousavi has visibly gained momentum in recent days and his exuberant supporters have flooded on to the streets of Tehran in huge numbers. The usual restrictions on dissent have been largely swept aside as the regime has loosened — or lost — its grip ahead of the election.

The election has also exposed the deep divide within Iranian society. Mr Mousavi’s candidacy has galvanised the urban middle-classes, and he also has the support of Mohammed Khatami, the popular reformist President from 1997 to 2005, and Hojatoleslam Rafsanjani, who was defeated by Mr Ahmadinejad in the presidential election of 2005.

Mr Ahmadinejad enjoys strong support among the rural poor and religiously devout, and is thought to have the backing of the Supreme Leader as well as the Revolutionary Guards and the volunteer Basij militia.

Yesterday, on a platform overshadowed by the blue dome and twin minarets of a mosque, the President addressed yet another gathering of frenzied, flag-waving followers of the sort that has convulsed Tehran, day and night, for two weeks.

“Bush said ‘You don’t need nuclear power’,” he declared. “My opponents asked why we needed nuclear power. They were echoing the words of our enemies. But Iran has become a nuclear state despite them, and it is a great and honourable achievement.” The true believers broke into deafening chants — “Ahmadi, we love you” and “Nuclear energy is our right”.

Rightly or wrongly, they adore this man who presents himself as the champion of the oppressed, the scourge of Iran’s corrupt elite with its loose morals, a leader who has stood up to the bullying West and restored their country’s pride. No matter that rampant inflation and chronic unemployment have made their lot much worse: he is their man,

By 8am a sizeable crowd had already gathered in front of the mosque, men on one side and women in heavy black chadors and tight black headscarves on the other. By 9am it had doubled in size. By 10am it stretched far into the distance.

The sun beat down, the temperature rose and bodies pressed together. A succession of speakers kept the faithful in a state of ferment. “Ahmadinejad for family values,” they chanted. “Ahmadinejad, you are the nation’s choice.”

Then they turned on Mr Mousavi, the President’s strongest challenger, and his links with Hojatoleslam Rafsanjani, who is considered the embodiment of the corrupt elite. “Mousavi, go,” they roared. “We don’t want an American sympathiser” and “Shame on you, Hashemi.”

The densely packed crowd surged back and forth like a human tide. The heat was suffocating. Sweat dripped from the men. It was well past 11 before Mr Ahmadinejad’s small figure appeared on the platform, and there was instant pandemonium. For five minutes Mr Ahmadinejad basked in the adoration, waving and raising his fingers in a victory sign.

Only snatches of his speech were audible above the din. “You see the lies, the accusations against me?” he asked, instantly painting himself as the underdog fighting a rotten Establishment. “Libel is a crime. They should be punished,” he shouted.

Mr Ahmadinejad pressed on. The Americans had threatened to imprison him if he went to the UN, he said. The Iranian Establishment urged him not to go, but he insisted. “Even if all the great powers and Zionists were ready with their guns I will take the message of the Iranian people to the world.” He accused his opponents of corruption, conspiracy and pillaging the nation’s wealth.

“Ahmadinejad is brave. Every night he catches a thief,” the crowd roared back in unison. They wanted revenge for their defeat four years ago, Mr Ahmadinejad went on, “but the people will consign them to the bottom of history with another big no”. The crowd erupted in cheers.

It was 90 minutes before Mr Ahmadinejad ended. “Oh Great and Compassionate God, sit the Iranian nation on the roof of the world.” As one, thecrowd responded: “Thank you, thank you, our President.”

As his followers drifted away, an old woman accosted The Times. “Tell the world what’s really happening in Iran,” she demanded. “Tell them there’s democracy here.”

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