"More than five million pilgrims" have flocked to the glittering shrine from other parts of Iraq as well as the Gulf, Iran, India and Pakistan since Friday, according to Adi al-Kazmi, an official at the site.

Many had travelled hundreds of kilometres on foot, by horse or even on camels over several days, caught outside in heavy thunderstorms that hit Iraq at the weekend.

Authorities had sealed off many of Baghdad's thoroughfares in recent days to allow pilgrims to reach the shrine, which is located in the city's northern Kadhimiyah neighbourhood.

The site features an intricately-designed mosque and includes Imam Kadhim's double-domed mausoleum.

Musa al-Kadhim, the seventh of 12 imams venerated by Shiite Muslims, died in 799 while in detention by Abbasid caliph Harun al-Rashid.

His death is commemorated yearly with a funeral reenactment that sees a mock coffin carried into the shrine by swaying pilgrims.

On Tuesday, huge crowds of praying visitors made their way to the shrine, passing dozens of tents erected by volunteers to provide free tea, sweets, and shelter.

A group of men dressed in black gathered in a tightly-packed circle and began rhythmically striking their chests as a sign of mourning.

Security forces have set up rings of checkpoints to thoroughly search all pilgrims, and no security incidents were reported.

In 2005, at least 965 pilgrims heading to the Imam Kadhim shrine died after a mass stampede along a nearby bridge that was sparked by rumours of a suicide bomber in the crowd.

Iraq declared victory against the Islamic State group in late 2017 after three years of fierce fighting that ravaged the country's economy.

It is now keen to demonstrate its newfound stability, particularly with religious tourism to its many Shiite sites.

Many of its religious tourists come from neighbouring Iran, which announced recently that it and Iraq would introduce reciprocal visa-free travel.