As they watched, the western delegations' nervous first impressions were quickly compounded. Mr Ahmadinejad was there to present Iran's case in the mounting international dispute over the country's nuclear programme, which the United States and its European allies suspect is a front for manufacturing an atomic bomb.

Adopting a tone of indignation, the Iranian president rubbished the charge. But he did so in a way that gave no quarter to western anxieties.

Iran had an 'inalienable right' to peaceful nuclear energy, he declared. In a pitch calculated to appeal to leaders of other developing nations, he accused the US and its friends of operating a system of 'nuclear apartheid' that discriminated against countries like Iran. And in a rhetorical rallying call to the Islamic world, he called for a UN investigation into which countries had given Israel the technology to develop nuclear weapons.

The broadside came against the backdrop of a breakdown in talks between Iran and the EU trio of Britain, France and Germany over the nuclear issue since Mr Ahmadinejad assumed the presidency last month. Days after he took office, Iran resumed uranium conversion activities, which had been suspended since last November pending negotiations. The move was an ominous step towards uranium enrichment, a process the west wants to prevent Iran from undertaking because it is essential for creating a nuclear bomb.

With the talks in limbo, the Europeans - backed by the Bush administration - used a board meeting of the International Atomic Energy Agency in Vienna this week to table a draft resolution referring Iran to the UN Security Council for flouting its obligations as a signatory to the nuclear Non-Proliferation Treaty. Iran threatened to retaliate if referred, possibly by leaving the treaty. With Russia publicly opposing taking the matter to the security council, the resolution was shelved - for the time being.

But the situation is at a dangerous impasse. US President George W. Bush has refused to rule out force, raising fears of an escalating confrontation that could lead to a military attack, as with Iraq.

The shape of things to come may hinge, as much as anything, on the character of Mr Ahmadinejad. In that case, the evidence suggests that Iran's new man is not for backing down. Even before he took office, Mr Ahmadinejad picked another hardliner, Ali Larijani, as the new head of the Supreme National Security Council, tasked with leading Iran's nuclear negotiating team. That move was in line with the charges of timidity and cowardice Mr Ahmadinejad had levelled against the negotiators while running for president.

Under Iran's Islamic political system, the president does not exert the same influence as, for example, an American president. The real power in Iran lies with the country's supreme leader, Ayatollah Ali Khamenei, a religious figure who had become increasingly concerned that, under Mr Ahmadinejad's reformist predecessor, Mohammad Khatami, the country was straying from the ideals established by the 1979 revolution.

A relatively benign public figure, Mr Khatami instituted liberalising social changes that loosened Iranian society's religious constraints. He had also alienated religious hardliners by talking about opening a 'dialogue of civilisations' with the west.

So as Iran prepared to go to the polls in June this year, the staunchly anti-western Ayatollah Khamenei determined that a different kind of president was needed. He was particularly anxious to find a figure robust enough to safeguard the country's nuclear ambitions in the face of international pressure. That meant preventing the presidency from being reclaimed by Ali Akbar Hashemi Rafsanjani, a former president and long-time political adversary now campaigning on a platform of reform and detente with America.

As election day approached, few observers saw Mr Ahmadinejad as a potential president. Despite being mayor of Tehran, the Iranian capital with 14 million inhabitants, his name was so little known that he did not appear in the latest edition of Iran's political who's who. In a field dominated by hardliners, Mr Ahmadinejad seemed the least likely to appeal to an apathetic electorate disillusioned by the perceived failures of Mr Khatami's administration. But in the closing stages of the campaign, something significant happened - albeit little noticed at the time. Mr Khamenei's representative on the Guardians Council, a powerful watchdog body charged with upholding Islamic values, called on voters to back a humble candidate.

This was code for Mr Ahmadinejad. As Tehran mayor, he had cultivated a man-of-the-people image by donning workers' overalls to help sweep the streets. He was also known to have taken packed lunches to work rather than accept the free canteen meals at Tehran city hall to which he was entitled. With the presidency at stake and resentment about economic inequalities widespread, Mr Ahmadinejad's working-class background as the son of a blacksmith suddenly became a selling point.

Word filtered down to mosques, from where last-minute voter mobilisation efforts were organised. Amid allegations of ballot-rigging and other irregularities, Mr Ahmadinejad finished a close second behind Mr Rafsanjani in a first ballot in which no candidate achieved the absolute majority needed to win outright. A week later, with the former president dogged by persistent rumours of financial scandals, Mr Ahmadinejad won power by a landslide.

The victory stunned foreign observers and many Iranians. But it should not have done. Mr Ahmadinejad was an Islamist after the supreme leader's heart. As Tehran mayor, he had enforced gender segregation in municipal lifts and clamped down on many cultural events fostered by the reformers, but anathema to hardliners. Previously, he had been an instructor to the basij, the youth volunteer movement that enforces the revolution's Islamic strictures.

In a dimension of his character that has provoked international controversy, he was also a dyed-in-the-wool revolutionary. Before the revolution that toppled the shah, Mr Ahmadinejad's political activities drew the attentions of the monarch's hated secret police, forcing his family to flee Tehran and go into hiding. He also visited Lebanon during the country's bloody civil war, actively supporting hardline Shi'ite militias.

But it is his alleged activities after the revolution which have triggered most contemporary debate. Days after his election, several survivors of the 1979-81 US embassy takeover claimed the new president had been a ring leader of the group which seized 52 American diplomats and held them hostage for 444 days.

'As soon as I saw his picture in the paper, I knew that was the bastard,' Charles Scott, one of the former hostages, said. 'He was one of the top two or three leaders ... the new president of Iran is a terrorist.'

It was a headline-grabbing allegation, but it was denied by Mr Ahmadinejad and by those who admitted being among the hostage-takers. A CIA inquiry has been unable to establish the truth one way or the other. What is certain, however, is that - whatever his role in the takeover- he was an active member of the student co-ordinating body behind it.

His subsequent career has given rise to further allegations. In the 1980s, Mr Ahmadinejad became a senior commander of the Revolutionary Guards, where he is said to have helped found the elite Qods brigade, a unit specialising in clandestine foreign operations.

That detail assumed a new significance in July when an Austrian member of parliament, Peter Pilz, claimed to have 'very convincing' evidence that Mr Ahmadinejad had travelled to Vienna in 1989 to deliver weapons later used in the murders of three members of the Iranian Kurdish opposition. Again, the claims were denied.

Whatever the truth, the clue to Mr Ahmadinejad's future behaviour may lie not in acts carried out in a revolutionary age long past, so much as with his status in the Iranian politics of the present. His election victory owed much to a populist appeal, fostered through railing against class discrimination, corruption and a desire to redistribute Iran's oil wealth.

The focus on oil raised the spectre of a previous Iranian populist, Mohammed Mossadegh, who as premier in the 1950s inspired the masses by nationalising the then British-controlled oil industry.

Mossadegh's actions roused popular nationalist sentiment, but eventually provoked a CIA-backed coup which overthrew him in 1953.

Blending oil with an assertion of Iran's nuclear rights may yield Mr Ahmadinejad a modern-day brew of populism and nationalism. The outcome may not be another coup. But confrontation with the world's only superpower is surely inevitable.