In Baghdad today, Mr Sadr will lead a rally uniting Shi'ites, who comprise 60 per cent of the country's population and wield real political power, and Sunnis, one-fifth of Iraqi's population and the former rulers under ousted dictator Saddam Hussein. A big turnout would cement his increasingly apparent position as Iraq's foremost political force.

Respect - or perhaps more likely, fear - for his authority was the reason Vice-President Adel Abdul Mahdi broke a weeks-long deadlock on Wednesday and signed a presidential decree allowing parliament to meet. He has set in motion the process for the formation of the country's first democratically elected government.

Mr Sadr's followers won 30 of the 275 parliamentary seats in the December election, and his backing enabled Prime Minister Ibrahim al-Jaafari to win the nomination of the Shi'ite bloc for a second term. But the outbreak of sectarian violence between Shi'ites and Sunnis after a sacred Shi'ite shrine was bombed on February 22 in the town of Samarra provided him with an opportunity he was quick to exploit.

Reprisal attacks against Sunni mosques around the country was swift. As many as 1,300 people have since died, pushing Iraq to the brink of civil war.

Observers contend many of the attacks against Sunnis were orchestrated by Mr Sadr's Mahdi Army militia, which operates in the Shi'ite slum of Baghdad's Sadr City and in Shi'ite strongholds throughout the country. The young cleric has denied such claims, saying he was in Lebanon at the time. Nonetheless, the worst of the violence began to die down when he joined moderate Shi'ite clerics in calling for peace.

Whatever his connection to the unrest, the message was loud and clear: Mr Sadr controls the streets and no deal to restore order would be successful without his support. Instantly, all of Iraq's political and religious leaders and even the US paid attention. None - not even the foremost Shi'ite cleric, Grand Ayatollah Ali al-Sistani - would at this stage dare challenge him.

That was apparent when Mr Abdul Mahdi finally caved in and signed the decree. As a result, a looming political crisis may also be stifled: Opponents of Dr Jaafari's candidacy on grounds he would be unable to form a government of unity due to his links to Mr Sadr may yet reconsider their campaign.

Dr Jaafari's confirmation as prime minister would be a major challenge to mainstream Shi'ite parties and the US and its allies, which had been hoping a stable government would allow them to gradually withdraw troops. He would undoubtedly give Mr Sadr's followers important posts in the new government, providing them with the power to make key decisions.

The cleric has already hinted at what that might mean. Returning to his home in the city of Najaf, south of Baghdad, last month, he suggested his militia not be disbanded and instead be given a formal role to work 'in co-ordination with the Iraqi government, army, police and people'. That would undercut efforts to create a professional Iraqi military and prevent foreign troops from leaving any time soon.

Further proving his vision for Iraq's future is markedly different from other leaders, he told followers that 'there is no such thing as Sunni or Shi'ite mosques - the mosques are for all Iraqi people and all Muslims'.

In the southern city of Basra, his militiamen have reportedly bombed shops selling alcohol and entertainment material considered improper, while women wearing what is deemed improper clothing are berated.

Mr Sadr also supports the hardline regimes in neighbouring Iran and Syria, which he visited to much acclaim last month. In Syria, he praised the victory of the militant group Hamas in Palestinian elections, saying: 'I hope it is the beginning of an Islamic awakening and that it will be the start ... of Islam's triumph in other Islamic countries.'

Despite such statements, Mr Sadr does not have the scholarly background in Islam that other leaders of his stature enjoy. His lack of religious education puts him at a middle rank within Shia Islam; only in a decade, assuming he fulfils his scholarly obligations, would he attain the top-most position, which would permit him to interpret the holy book the Koran and hand down edicts.

Nevertheless, he has a loyal following of thousands of supporters, many of whom vow they would die for him. Mostly from the slums of Iraq - his lack of religious credentials have made better-educated, middle-class Iraqis wary - they refer to him as 'al-Sayed', or master, and he wears the black turban reserved for descendents of Mohammed.

His pedigree to lead is flawless. His father was Iraq's top Shi'ite cleric, but fell out with Hussein's regime and was assassinated along with Mr Sadr's two elder brothers in 1999. The same fate befell his uncle, a prominent ayatollah and philosopher, in 1980. His grandfather was Iraq's prime minister in 1948.

With Hussein's removal by the US-led invasion in 2003, Mr Sadr began his rise to prominence, first setting up his militia in defiance of American orders, and then a newspaper that was temporarily shut down for allegedly inciting anti-US violence.

The radical cleric has led uprisings against US forces at Najaf, prompting the intervention of Ayatollah Sistani. He has also evaded an arrest warrant over the murder of a moderate Shi'ite leader killed just two days after the fall of Baghdad.

Mr Sadr is not a great orator; he stands woodenly before his supporters, reading his speeches in a hesitant, often awkward, manner. He makes up for the lack of this skill by showing compassion.

'I feel for you my beloved demonstrators and find that you tire yourself greatly,' he said in calling off street protests in April 2004. I am with you in heart and body and will never leave you to face difficulties alone.'

Such sentiments are met with gusto at rallies. His mostly young supporters wave giant photographs of his father and chant their readiness to die for him. Such devotion could take him a long way in Iraq. He has manipulated himself into a strong position and the rewards could be many.

But Iraq is also going through tumultuous times and what may seem certain now could well be tragedy tomorrow.