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The act is to remember the death of Hussein, the son of Ali and grandson of the Prophet Mohammed, who remains an iconic figure in Shia-Islam.

His brutal death in Karbala led to an annual commemoration in the city, where some pilgrims beat their chest and mourn his demise.

It is a ceremony that marks the schism in Islam between Shia and Sunni Muslims, and also the division between the two groups in conflict-ravaged Iraq.

With millions on the street, the event is an obvious target for Islamic State group militants who consider Shias to be heretics and are waging a war against the Iraqi government and Shia militias in Mosul.

Military setbacks for the jihadi group has led IS militants to resort to so-called soft targets - targeting civilians with suicide bomb attacks and shootings. 

On Sunday, security forces and militias were on the streets in their thousands to provide what security and reassurance and can for the pilgrims who could number 20 million in the coming days.

Many more are fighting against IS in the north of the country or providing security or directing traffic along the busy thoroughfares leading to the city.

This includes millions of Iranians pilgrims, with some well-known figures such as Republican Guard commander Qasem Soleimani spotted in the city, along with his generals, who are fighting IS in Iraq and rebels in Syria.

"We are on maximum alert," Staff Major General Qais Khalaf Rahaima, the head of the security command responsible for the area, told AFP.

Many of the pilgrims have marched hundreds of miles to reach Karbala - particularly from the capital Baghdad and Basra in the south.

Along the way, Iraqis have opened up their homes, given out food and water, or just kind words of support to the weary travellers.

Arbaeen, which means "forty" in Arabic, is an observance that peaks on the 40th day after the anniversary of Hussein's death. 

Arbaeen would be a prize target for the IS militants owing to the symbolism of the event, but so-far they have been unsuccessful.

In previous years they have left booby-trapped dolls on the sides of the road to kill pilgrims, particularly children.

However, thankfully they were unsuccessful and the event remains one of sharing, giving and mourning of the late Hussein.