Ali Khamenei: The real power in Iran

British diplomats wrestling with the crisis caused by Iran's capture of 15 British sailors and Marines in the Gulf nearly 10 days ago have been directed to a recent speech by the country's supreme leader, Ayatollah Ali Khamenei.

"In case the enemies of Iran intend to use force and violence and act illegally, without a doubt the Iranian nation and officials will use all their capabilities to strike the invading enemies," he said. "We can also carry out illegal actions."

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The words have been interpreted as the green light for the Revolutionary Guards, the more ideological wing of the military, to take action against the British military personnel. Certainly they carry more weight than if Mahmoud Ahmadinejad had said them - Iran's firebrand President may make the headlines in the West, but at home his position is very much contingent on Khamenei's support.

After Ahmadinejad's wild rhetoric and economic mistakes were blamed for increasing the pressure on Iran in January, he was unofficially reined in, and has played a much quieter role in the past two months. The supreme leader may well have been behind this, though as usual in Iran's confusing political system, nobody can be quite sure.

Only Khamenei has both the legal powers and the personal authority to impose his will on the other parts of the state. "Everyone behind the leader," shouted a young man with fierce eyes at Tehran's Friday prayers. "We will follow the leader to victory." But Khamenei does not command universal obedience. He sits instead at the fulcrum of Iranian politics, where elected politicians hash out policy with unelected apparatchiks, Revolutionary Guards and turbaned religious jurists.

The expansive white beard and huge glasses create an impression of extreme old age, but Khamenei is only 67. Despite long-running rumours of ill-health, he looks unlikely to leave the Iranian political scene any time soon. If the West is to successfully navigate its numerous disputes with Iran - which, as well as the crisis with Britain, include the nuclear issue, accusations of terrorism, human rights, the future of Iraq and sanctions - it must read him correctly.

At the heart of Iranian politics since the revolution, as Friday prayer leader, party apparatchik, president and, from 1989, as supreme leader, Khamenei has had an intimate involvement with every aspect of the Islamist state. He appoints the heads and senior members of the judiciary, the Revolutionary Guards and the Guardian Council, which controls who can run in elections. His speeches are rarely specific, but they set the tone of policy.

As president he ran the Iran-Iraq war alongside the then speaker of parliament, Akbar Hashemi Rafsanjani, developing close ties to the top military leaders. And as supreme leader he has fought to maintain the primacy of his position after initial challenges to his religious authority and reformists' later incursions.

Ali Khamenei grew up in the great shrine city of Mashhad, near the borders of Afghanistan and Turkmenistan. The son of a cleric, his formative years were spent in the vaulted iwans and cloistered courtyards of great mosques, sitting cross-legged on exquisite Persian carpets, absorbing the religious teachings of his elders.

But his upbringing did not shelter him from Iran's tides of violence. At 14 he would have seen the rioters running in the streets as British and American agents brought down the populist Prime Minister, Mohammed Mossadeq. As a young student in Qom, the great seminary city, he took part in risings against the Shah. Radicalised and revolutionary, he was imprisoned several times before Ayatollah Ruhollah Khomeini returned from exile in 1979 to usher in Islamic rule.

In the wake of that upheaval, a bombing campaign shattered Iran's new leadership. Khamenei narrowly escaped: a bomb hidden in a tape recorder at a press conference crippled his right hand.

It is impossible to tell Khamenei's story without telling that of Khomeini, the father of Iran's revolution. There is a revolutionary mural - one of many in central Tehran - showing the leader's face looking up towards the future and contained entirely within the stern, patrician features of his predecessor. "The path of Khamenei is the path of Khomeini," reads the slogan.

In offices, shops, restaurants and hotel lobbies the two faces sit side by side, each in its own frame. And on television, endless montages of the revolution show the two men praying together, the one posthumously conferring legitimacy on the other.

Khomeini's death left the young republic with a quandary. His charisma had been pivotal to the revolution: how could it continue without him? The blend of political leadership and religious authority embodied in Khomeini was rare. None of the grand ayatollahs seemed suitable, so minds turned to the younger, more politicised clerics.

When the Assembly of Experts, an elected body of old clerics and Islamic jurists, elected Khamenei supreme leader, Rafsanjani replaced him as President. The pair have dominated Iranian politics in the 18 years since, both as rivals and as partners.

But Khamenei could never hope to enjoy the instinctive support given to Khomeini for his charisma, his supreme religious authority and his leadership of the revolution. When Khomeini had graced a cause or a person with his favour, the country followed. Where he criticised, heads rolled. The new supreme leader now possessed many of Khomeini's powers, but had to negotiate to push them through.

The very highest clerics in Shia Islam are accorded the title Source of Emulation. Hundreds of thousands,even millions of devotees, base their religious practices entirely on their pronouncements. Khomeini was the most eminent such cleric of his generation, followed by Shias across the world. He could mobilise a mass of religious support that Khamenei could not.

The system Khomeini created is called velayat-e faqih: rule of the jurist. Shias recognise 12 legitimate heirs of the Muslim community left by Mohamed. The twelfth disappeared in the ninth century, but Shias believe he still hides somewhere in the cosmos and will one day return.

Theologians used to say that until then, all temporal power was unlawful. Khomeini's innovation was to see Shia jurists as the imam's natural deputies. This revolutionary idea emerged after a century of growing clerical politicisation and as the Shah's royal state began to collapse.

Khamenei and Rafsanjani have now run Iran under the system for nearly twice as long as Khomeini. They have survived numerous upheavals with a combination of shrewd political management and ruthless intolerance of threats to the system.

But as the pressure on Iran increases, questions remain over his understanding of the West, which he is not known to have visited. If he did instigate this most recent confrontation, it is unclear whether he expected Britain to react so forcefully. Ali Khamenei is a leader unlike any other, but the rules of politics still apply: a single miscalculation can change everything.