Tens of thousands of Iraqi Shia faithful are making their way towards the holy city of Karbala on a pilgrimage that was banned under the government of Saddam Hussein.
The highway out of Baghdad is full of pilgrims
The pilgrims are making the journey to commemorate the anniversary of the death in battle of Hussein, a grandson of the Prophet Muhammad, in 680.
The festival, culminating with a mass gathering at Hussein's grave on Tuesday and Wednesday, is of enormous religious significance to Shia Muslims.
But the BBC's David Bamford says Ayatollah Muhammad Bakr al-Hakim's call for the festival to go ahead for the first time in 25 years is of political significance too - a chance for a show of force both to the Americans and to rival Shia leaders.
In an address on Friday, the ayatollah called on the faithful to converge on Karbala to express their opposition to foreign domination, and their support for the establishment of a free and just government.
Shia will converge on Hussein's holy tomb
The ayatollah leads the main Shia group, the Supreme Assembly of the Islamic Revolution in Iraq, that boycotted an American-sponsored political gathering at Ur last Tuesday.
He has been in exile in Iran for over 20 years, but says he will return to Iraq when it is appropriate.
In the meantime, he has sent his younger brother and deputy, Abdul Aziz al-Hakim, who supporters say has been welcomed by massive crowds as he passed through the towns of Kut and al-Amarah.
Shia leaders expect over a million to heed the ayatollah's call, creating a security headache for coalition forces.
The US military spokesman at Central Command in Qatar, General Vincent Brooks, said coalition forces were aware of the Karbala pilgrimage and would ensure its security.
But our correspondent says there is little military presence along the route, as the Americans seem to have chosen not to interfere in the hope it will pass off peacefully.
Along the main route between the capital Baghdad and Karbala, traffic has been slowed by the crowds and stalls line the roadside selling food to pilgrims.
The worshippers hail from all over Iraq - Shia Muslims make up about 60% of the population, but they were sidelined under Saddam Hussein who tried to promote the Sunni minority.