Three weeks ago Mir Hossein Mousavi was chastising women for appearing at his rallies with “bad hijab” — wearing their headscarves too far back.

At that point he was a mere presidential candidate seeking to attract disaffected conservatives as much as moderates and reformists. His single most daring act was to hold hands in public with his wife, Zahra Rahnavard.

Today, at considerable personal risk, the 67-year-old artist, architect and grandfather pops up at massive rallies, speaks briefly into a hand-held loudhailer, then vanishes again.

He sends letters to the Guardian Council, the election commission and other institutions, denouncing their “disgusting” attempts to rig the election. He openly defies Ayatollah Ali Khamenei, the Supreme Leader, who demanded on Friday that the demonstrations end.

It is an astounding transformation. A former prime minister and creature of Iran’s political establishment since the revolution of 1979, Mr Mousavi now threatens its very survival.

Despite threats of violence and mass bloodshed, and intense pressure from Iran’s Supreme Leader and others, he has refused to accept what he sees as a rigged election and betray the millions of protesters who have taken to the streets in his name.

“Be sure that I will always stand with you,” he declared in a remarkable statement to his supporters posted on his website at the weekend.

He was not a counter-revolutionary, he insisted. He was seeking to save the revolution from those who are subverting its values and leading Iran towards dictatorship with their “deception and lies”.

Mr Mousavi began his political career as a hardliner. He played an active part in the toppling of the US-backed Shah 30 years ago, and was close to Ayatollah Ruhollah Khomenei, the architect of that revolution. As prime minister from 1981 to 1989 he was an early supporter of Iran’s fledgeling nuclear programme, tolerated little dissent and often clashed with Mr Khamenei, who was president at the time. In those days he was more conservative than Mr Khamenei, at least on economic matters.

Mr Mousavi spent the next 20 years pursuing his art and architecture, though he remained a member of two top-level government committees.

Although he decided to stand this year because he considered Mahmoud Ahmadinejad’s presidency dangerous, his candidacy was approved by the Guardian Council, the Islamic watchdog, and his platform was hardly radical — greater social and economic liberalisation, and a less abrasive foreign policy.

Cautious, soft-spoken and distinctly lacking in charisma, he never intended to challenge the political system.

It is a measure of the regime’s disastrous handling of the election and the contempt with which it has treated the Iranian people that it has managed to turn this quiet intellectual into the opposition’s champion.

Outraged by what has happened, and doubtless encouraged by his redoubtable wife, Mr Mousavi’s resolve has hardened. He has come to realise that the regime is far more tyrannical than he realised. If he can destroy it, he will.

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