The roads leading to Karbala are lined with pilgrims. The columns, sometimes five or six abreast, stretch for at least 100 kilometres (60 miles).
Shia Muslims held a vigil in Karbala ahead of the pilgrimage
The worshippers sing as they march to their holy city, flinging their arms in the air and then beating their chests in prayer.
Old women and children as young as four or five trail behind. They pause to sit down away from the hot tarmac, in what little shade they can find.
Some lie, lifting tired feet in the air, to rest them against trees and buildings.
On the road I met Mudel Hussein, an English teacher.
"Before this time it was impossible to do this," he told me, "because Saddam Hussein didn't let us come freely.
"He thought that we were against him."
And he added: "I feel very happy today, because today we can make the journey openly.
"Saddam Hussein was a nightmare and only George Bush helped us to get rid of him.
"For the future, I think it is good if everyone can live their life freely, Shia, Sunni, everyone."
In the open desert, water trucks are parked by the roadside for the pilgrims.
They carry flags and banners, some with inscriptions from the Koran, others praising Hussein, the grandson of the Prophet Mohammed, whose martyrdom they are coming to commemorate.
It was Hussein's death which caused the split between Shia and Sunni Muslims.
For the past 25 years Shias were banned from making this pilgrimage.
Saddam Hussein's security forces used to set up checkpoints on the roads. They would shoot at those trying to make the journey.
So the vast human tide heading for Karbala is both an expression of faith and a celebration of their new-found freedom.
In Karbala itself the crowds are swelling, as hundreds of thousands of Shias gather ahead of the main festival on Tuesday.
The avenue between the city's two biggest shrines is becoming a heaving sea of people, the first gathering like this in at least three decades.