Scorning his constitutional role of standing above party politics, Ayatollah Ali Khamanei, Iran’s Supreme Leader, threw his full weight behind Mahmoud Ahmadinejad yesterday, outlawed any further questioning of his election victory and warned Iran’s protesters that he would unleash the Revolutionary Guard and Islamic militias on any future demonstrations and hold opposition leaders responsible for the bloodshed.

His uncompromising stance has set the stage for a showdown. If, as planned, the vast crowds of angry demonstrators throng the streets of Tehran and other cities again today, with or without the blessing of Mir Hossein Mousavi, the opposition contender cheated of his votes, the response could be gunfire. Ayatollah Khamanei has revealed the true face of the hardline clerics determined to remain in power: paranoid, mendacious, fearful and prepared to kill. Blaming America, Britain and Western media for inciting the rioters, he insisted in his Friday sermon that Mr Ahmadinejad, his protégé, had won an “absolute victory”. Was it possible, he asked with the risible logic of the Big Lie, that an election could be rigged when the difference was 11 million votes?

His stance has appalled the outside world, inured as it is to official ravings in Tehran. The European Union expressed unanimous dismay yesterday at his threat of a crackdown. Britain went farther: Gordon Brown spoke of the “repression and brutality” of the past few days, and said that it was up to Iran to prove that it could respect basic rights if it wanted to rejoin the international community. The Foreign Office summoned the Iranian Ambassador to protest at remarks by Ayatollah Khamanei singling out Britain as “the most treacherous” of Iran’s enemies.

Britain’s anger is fully justified, and Mr Brown did well to express it. But there is a more powerful voice that would have far more resonance in Iran. President Obama said at the start of the demonstrations on Monday that the world was inspired by the outpouring of political dissent. Last night he restated his concerns, saying, after the Ayatollah’s speech that he was alarmed by the “tenor and tone of the statements.

“How they approach and deal with people . . . will send a pretty clear signal to the international community about what Iran is and is not.” This was a sensible formulation. It makes clear where America’s sympathies lie. And it undercuts the expected claim by Iranian hardliners that the US is meddling. Mr Obama enjoys huge popularity around the world, especially in Iran. He is ideally placed to rekindle faith in the US as a champion of democratic values and uphold the cause of liberty. But there is a difficulty, which he recognises, in throwing his weight behind any of the contenders in Iran. Any such statement would be seized on by the clerical establishment as proof that the US was threatening intervention and that Iran must redouble it vigilance against the “Great Satan”. Any overt US support for Mr Mousavi — himself no pro-Western liberal — would be the kiss of death. As Mr Obama said, “the United States can be a handy football”.

The US President is a skilful orator who understands soft power. He has already shown, in his speech in Cairo, how he is able to inspire those yearning for a more democratic and open system without yoking their cause to American policies. Now it is time to do the same over Iran. It is time to speak out on the broader theme of liberty, human rights and respect for democratic process. The US does not need to pick a winner in Iran. But it does need to hold up a light for freedom. The world expects it. And so do millions of Iranians. Nothing sustained Anatoly Shcharansky and other jailed Soviet dissidents as much as President Reagan’s vocal support. Mr Obama needs to offer that support to a jailed nation.