Thirty-one years ago this week, Ayatollah Khomeini returned to Tehran after 15 years in exile. The anniversary is usually marked by triumphant rallies. Not this time: protesters are planning mass demonstrations against a regime they say has betrayed Islamic ideals.

For three decades, the image of Ayatollah Ruhollah Khomeini arriving on Iranian soil to a tumultuous homecoming after 15 years in exile has been a centrepiece of Iran's revolutionary iconography.

It is an event best captured in a famous picture of the late spiritual leader being gently led down the steps of an Air France jet by a man dressed as a pilot or an air steward. The picture embodies the heady mixture of pride, compassion and religious hero-worship the revolution is supposed to evoke among Iranians.

Khomeini was returning to be hailed as a saviour by his fellow countrymen after a wave of popular uprisings that had toppled the regime of the western-backed shah, Mohammad Reza Pahlavi. His guide was playing a mere walk-on part in the historic drama that engulfed Iran that day in February 1979.

But last week, at the start of the annual Fajr festivities marking the revolution's anniversary, that image was the subject of a strange story that seemed symptomatic of the increasing uncertainty surrounding the country's revolutionary legacy, amid the continuing turmoil over President Mahmoud Ahmadinejad's disputed re-election.

The Islamic Revolution Documentation Centre, a state body linked to a pro-government MP, claimed on its website to have traced Khomeini's chaperone as one Gerard Jean Fabian-Bataouche, who it said was living almost destitute in Tehran. The report, based on an interview with Fabian-Bataouche, described him as a former French policeman of Algerian origin who had been Khomeini's personal bodyguard while he was living in the village of Neauphle-le-Château, outside Paris, waiting for the shah to fall in the months before the revolution.

Having taken a liking to the man after learning that he had an Iranian wife and spoke Farsi, Khomeini had invited him to be on board his triumphant flight to Tehran. Fabian-Bataouche had remained in Iran afterwards but had fallen on hard times. He was said to be homeless and forced to flit from one friend to another in an endless quest for a place to sleep.

It seemed an improbably shabby postscript to an association with the ­revolution's founding father. Predictably, the story was immediately denounced as a hoax and within a day, the Islamic Revolution Documentation Centre removed it, citing "serious doubts" about Fabian-Bataouche's authenticity.

True or false, the fact such a tale even saw the light of day betrayed an uncharacteristic lack of official sure-footedness as the revolution approaches its 31st anniversary. The prospect of revolutionary festivities is usually a cause for triumphalism among the Islamic republic's establishment. Instead, with the storm over Ahmadinejad's hotly disputed poll victory last June refusing to abate, it appears to be making them jumpy.

That was evident last Sunday when Iran's ambassador to France, Mehdi Mir Abu Talebi, was involved in an unseemly scuffle with French police as he arrived on an annual pilgrimage to Neauphle-le-Château. Angered by the presence of anti-government demonstrators, Abu Talebi had to be restrained by police officers, after allegedly punching one of them in the chest, according to one account. He was said to have been released only after a colleague told police that he had diplomatic immunity.

Such nervousness will be even more apparent this Thursday when the country's leaders gather to mark the revolution's final triumph, when the shah's armed forces declared themselves neutral and allowed Khomeini's followers to take control. Previous anniversaries have been marked by triumphant mass rallies in front of Tehran's Azadi monument. By contrast, this year's event threatens to be a bloody battleground witnessing a re-enactment of the extreme violence between security forces and opposition supporters that has marked several state-sanctioned public gatherings since Ahmadinejad's allegedly fraudulent election victory, which has been the trigger for a cycle of anti-regime protest. At least eight people – and possibly more – were killed when government forces fired on protesters in Tehran and other cities during the Shia Ashura ceremony in December.

The scene for renewed confrontation was set last week when the two leaders of the opposition Green Movement, Mir Hossein Mousavi and Mehdi Karroubi, urged their followers to turn out in force on Thursday to protest against the brutal crackdown that has seen at least 80 people killed, thousands arrested and two executed for being part of what regime hardliners describe as "sedition". Another nine have been given death sentences after being convicted of moharebeh ("fighting against God").

The government has vowed to suppress further protests with even greater force than before, while a hardline cleric, Ayatollah Ahmad Jannati, secretary of the powerful guardian council, has called for more executions to deter further protests. "There is no room for Islamic mercy," Jannati told a recent Friday prayer sermon at Tehran University.

At the heart of such confrontational rhetoric is a battle for the very soul of the revolution and what it was designed to achieve. While supporters of the supreme leader, Ayatollah Ali Khamenei, swear loyalty to the concept of velayat-e faqih (leadership by an Islamic jurisprudence) devised by him, Mousavi and Karroubi have been suggesting with increasing boldness that the revolution has failed to free Iranians from tyranny. In fact, they claim, "leadership" by an Islamic jurisprudence has merely instituted a new form of political bondage.

Last week, in an unusually hard-hitting interview on his website, Kalemeh, Mousavi – who served as prime minister under Khomeini in the 1980s – accused the authorities of filling the prisons with "the most sincere and devoted sons of this nation" and warned that the Islamic regime was in danger of becoming a worse dictatorship than the shah's regime. "Dictatorships in the name of religion are the worst kind of dictatorships," he said.

"In the first years of the revolution, people were convinced the revolution had completely destroyed all those structures through which despotism and dictatorships could be recreated," Mousavi went on. "I was one of the people who believed this. But today, I do not believe it any more. Today, we can identify those very structures that have led to despotism… Therefore I don't believe that the Islamic revolution has achieved its goals."

To Khamenei, who succeeded Khomeini as leader on the latter's death in 1989, such talk is tantamount to heresy. Having publicly endorsed Ahmadinejad's tainted election victory, Khamenei has committed himself to a course of depicting the Green Movement as puppets of a western-backed conspiracy bent on toppling the Islamic system in a "velvet revolution".

In recent weeks, Mousavi and Karroubi had tried to bridge the schism between them and Khamenei by hinting at a compromise that would head off a final cataclysmic confrontation. They had called for changes guaranteeing free elections, a free press and the freedom to demonstrate, while demanding the unconditional release of all political prisoners. Crucially, in a step that implies acceptance of Ahmadinejad's tenure as president – at least for now – they had also demanded that the government be held "accountable" for its actions. But these olive branches were offered before Mousavi's latest statement, which further sharpened the divisions.

In this battle of wills, each side presents itself as the heir to Khomeini's legacy. The "imam", as he is called in Iran's revolutionary lexicon, is revered for basing his movement on five pillars: people's sovereignty, freedom, independence, social justice for the weak and deprived, and Islam. Mousavi and Karroubi – protesting fealty to the Islamic republic formed in Khomeini's name and anxious not to be associated with more radical calls for a secular state – insist that their demands would have met with the revolutionary leader's approval.

But others who were present at the Islamic republic's creation take a less sanguine view. Abolhassan Bani-Sadr, the Islamic republic's first president, who fled Iran in 1981 after Khomeini ordered parliament to impeach him, says the late ayatollah would have approved of Khamenei's confrontational approach to the would-be reformers.

"He would have said 100 times well done to Mr Khamenei because the very regime he established is being continued," Bani-Sadr told the Observer from Versailles, where he lives in exile, under 24-hour police protection. "Khomeini's aim was to establish and hold despotic power in Iran and this has come true. Such a regime is now ruling and its official language is lying and violence, its mission is creating crisis and it considers the economy as something for donkeys. Mr Khomeini considered things that way."

The pre-revolutionary ideals expressed by Khomeini during his exile in France were, according to Bani-Sadr – who observed him at close quarters both then and after his return to Iran – mere window-dressing for public consumption, which he always intended to dump as soon as he acquired power. He said: "[Khomeini] said several times: 'I said some things in France due to ­expediency but I don't consider myself bound by them at all. If required, I say one thing today and something opposite the day after.' He had made a commitment in front of the world that he later very easily disregarded."

None of the goals publicly set by Khomeini have been achieved, said Bani-Sadr, who vividly recalls the revolutionary leader being greeted by "a sea of happy, hopeful people full of trust" on his arrival from France. Instead of transferring sovereignty to the people, as promised, power was invested in Khomeini's brainchild of velayet-e faqih, whose survival depended solely on the support of clerics.

Another former insider, Mohsen Sazegara, believes the regime's current hardline character was an inevitable outcome of the radical Islamist ideology that drove the revolution. "The pillar slogans of the revolution were freedom, independence and Islamic republic," said Sazegara, a one-time deputy prime minister and founder of the revolutionary guards. "But the applied result of that revolutionary ideological brand of Islam was religious despotism. As student activists under the shah, we all hoped to have an independent country that was modernised and free. We didn't understand in those days that this could not come out of that revolutionary ideology. The religious jurisprudence leadership in Iran set up under velayet-e faqih is a system that gives all authority to one man without any accountability, so very simply the result will be a dictator."

Sazegara, who has been issuing nightly videos from Washington calling for non-violent protest in support of Mousavi, said another brutal crackdown against protesters at Thursday's anniversary events could presage the regime's collapse. It would be the culmination of a four-pronged Green movement strategy aimed at delegitimising Ahmadinejad's government, strengthening popular resistance, creating splits in the regime and paralysing the government through civil disobedience.

"If the people show on Thursday that they aren't afraid and come on to the streets, then we can definitely say that all these four targets have been met," Sazegara said. "If they start beating people again, it will lead to the final stage, when the regime can be overthrown.

"Nobody can foresee when the final collapse might happen, but it could take just an accident or a demonstration, and maybe the same thing will happen to Ahmadinejad as happened to [overthrown Romanian] President Ceaucescu."

For Bani-Sadr, the regime's demise is only a matter of time – and no new revolution is required to replace it. "This regime doesn't need a revolution to collapse," he said. "It is only left with one pillar, the clergy, and part of that is already broken, since many clerics have distanced themselves from it. There's no further revolution needed because we haven't yet given up the one of 1979."

Additional reporting by Noushin Hoseiny