As a sign of how Iran’s political establishment has sundered, it could hardly have been more stark: the son and daughter of Ali Akbar Hashemi Rafsanjani, the former President, were barred from leaving the country yesterday for allegedly inciting unrest.

Until recently, Hojatoleslam Rafsanjani was one of the wealthiest and most powerful men in the country. Today he is on the wrong side of what many analysts consider an ultraconservative coup d’état. His children are not alone: hundreds of relative moderates and reformists, many of them former revolutionaries, have been detained in the past few days.

It is a coup that appears to have been planned well in advance. British officials believe President Ahmadinejad had prepared for the possibility of losing to his closest rival, Mir Hossein Mousavi, by packing the Interior Ministry — which conducted the count — with loyalists.

In the event, Western officials believe Mr Mousavi won last Friday’s poll, but not by a wide enough margin to avoid a run-off. It is at this point that Ahmadinejad loyalists put Plan B into action: falsifying the results and declaring their mentor the winner.

Their scheme could yet be wrecked by the protests that are threatening the very foundations of the regime. On one side of this trial of strength are Mr Mousavi, a former Prime Minister, and two former presidents, Hojatoleslam Rafsanjani and the popular reformist Mohammad Khatami.

They have the increasingly robust support of Mehdi Karoubi, a former parliamentary speaker who was the most liberal candidate in the election, and of much of Iran’s merchant class — not to mention the millions of protesters who have flooded on to the streets this week. Ali Larijani, the parliamentary speaker, has kept a foot in the Mousavi camp by condemning some of this week’s repression.

They are pitted against President Ahmadinejad, much (but not all) of the 125,000-strong Revolutionary Guard, tens of thousands of fanatical Islamic volunteer militiamen called Basiji, and a state machinery including the media and the judiciary, which is packed with his supporters.

And, crucially, the Supreme Leader, Ayatollah Ali Khamenei, the shadowy septuagenarian who is the most powerful man in Iran. One of the mysteries of recent Iranian history is why Ayatollah Khamenei backed the unlikely and little-known figure of Mr Ahmadinejad for the presidency in 2005. One theory is that he wanted a non-cleric in the post to forestall potential rivals to his eldest son, Mojtaba, eventually succeeding him as Supreme Leader.

Another mystery is why those who rigged last Friday’s election — undoubtedly with the blessing of Ayatollah Khamenei — announced that Mr Ahmadinejad had won by such a landslide, when few would have questioned a narrow victory. Again there is a theory: it was the brainchild of Ayatollah Mesbah Yazdi, Mr Ahmadinejad’s mentor and one of the most hardline clerics, who dislikes democracy. Give Mr Ahmadinejad a big enough mandate, the theory goes, and he could turn Iran into a true theocracy.

Whatever the reason, Ayatollah Khamenei rushed to endorse Mr Ahmadinejad’s victory, calling it a “divine assessment”. He reckoned, however, without the anger of the cheated and insulted populace. As the protests grew, he played for time in the hope that the fury would subside. He asked the Guardian Council, a body of 12 senior clerics that he controls, to investigate claims of irregularities — a ten-day process.

It was a ruse that manifestly has not worked. The protests have merely gathered momentum, the Supreme Leader’s pleas for calm have been ignored and his authority has been seriously damaged.

His sermon at Friday prayers in Tehran today will be scrutinised minutely for any sign that his support for Mr Ahmadinejad is weakening. The Supreme Leader is “in a very, very difficult bind”, says Ali Ansari, the director of the Institute of Iranian Studies at St Andrews University. “The minute he looks weak he will give encouragement to the opposition; too tough and he will just aggravate the problem.”

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