When Ayatollah Khomeini returned to Tehran, Hamid Reza Jalaiepour knew exactly what to do. A student activist, he had spent spells in the Shah’s prisons for handing out revolutionary pamphlets and organising his classmates to protest against the authorities. Alongside 1,500 comrades, he now headed to the airport determined to join a guard of honour to protect the returning cleric from counter-revolutionaries.

"It was so exciting," he remembers. "So many people came to the airport that we lost each other in the crowd. It was like a sea of people, and I was only able to glimpse the Imam through a car window."

Close to three million people were believed to have thronged the streets on that winter day, bringing to an end the rule of Mohammed Reza Shah, who had ignominiously fled a fortnight earlier.

Now, as Iran prepares to celebrate the 30th anniversary of its revolution on Tuesday, many of those who ran through the streets of Tehran in 1979 are disappointed with the results.

As students, their behaviour shook the world, bringing down the American president, Jimmy Carter, and unleashing a wave of revolutionary fervour that utterly changed the Middle East – inspiring militants in Lebanon, the Occupied Territories and Iraq.

Almost overnight, the West’s most steadfast ally in the Muslim world had become a violent and volatile enemy, where mass crowds raised their fists to chant "death to America". The students, mullahs and intellectuals who packed the streets were convinced that they had struck a blow for freedom against the imperialist might of the world’s strongest powers.

"The revolution was very genuine and popular," believes Ebrahim Yazdi, who served the revolutionary state as foreign minister and deputy prime minister before falling out of favour. "It was unavoidable because of the policies of the Shah and the foreign countries who supported him."

Yet the Islamic Republic created by Mr Yazdi and his comrades failed to live up to the dreams of a Muslim democracy, in which sagacious ayatollahs would stand as guardians of the democratic wishes of the people.

"What is happening now is a disaster," says Mahmood Delkhasteh, one of the first young soldiers to heed Khomeini’s call to desert the Shah’s army and join the revolution. "Many people regret participating."

Within months of the revolution, the euphoria had evaporated as the rival factions began a brutal battle for control of the country, which ended with a repressive state that imprisoned and executed thousands of political prisoners – including many of the revolutionaries themselves.

Still, for all the triumphalism of the grainy footage from 1979, broadcast incessantly over the past few days on state television, 30 years later Iran is approaching a crossroads. In the broad, plane-lined boulevards and spacious parks of Tehran, covered last week in a thick blanket of snow, the children of the revolution are struggling to square the aspirations of 1979 with the country they have inherited.

Every Friday, the revolutionary faithful still gather at Tehran University to raise their fists against the United States, the "Great Satan", but in the wider city the fervour has dimmed. Behind closed doors, families sit and watch Iranian pop stars strutting on television, beamed in via illegal satellite dishes from Los Angeles – dubbed Tehrangeles by its hundreds of thousands of Iranian immigrants.

On street corners they buy pirated versions of American DVDs while their children – two thirds of Iranians are under the age of 30 – apply in large numbers for visas to work and study in the West.

Still proud of their country’s struggle to overthrow the Shah, a man seen by many as a foreign-backed puppet, they despair over their current lack of political freedom and the chaotic state of the economy.

This summer, they will decide the future of Mahmoud Ahmadinejad, the firebrand president who sought to revive the years of revolution with a blend of Islamic fundamentalism, Left-wing economics and anti-American rhetoric. He will be challenged in a general election by Mohammed Khatami, his predecessor as president, who for eight years strove against powerful conservatives to make the country more democratic and open.

The contest comes at a pivotal moment, as President Obama offers Iran a chance to resolve the explosive confrontation over its nuclear programme through dialogue. If successful, the American president’s efforts could lead to rapprochement. If they fail, however, Iran and the US could be on the path to war as Western countries try to stop Tehran from developing the technology to build a nuclear bomb.