For Shia Muslims, Hussein is a symbol of rebellion against oppression. They believe he gave his life to keep true Islam alive

Iranian pilgrims on the outskirts of Mehran head towards the border with Iraq. Since the fall of Saddam Hussein, the number of Iranians making the pilgrimage to the Iraqi cities of Najaf and Karbala to participate in Arbaeen has been steadily increasing

Every year a number of Iranian Shias are caught trying to cross the Iraqi border illegally to participate in Arbaeen. Two young pilgrims prevented from crossing the border at Mehran, owing to a lack of passports, decided to remain there for the duration of the pilgrimage and polish the shoes of other pilgrims free of charge

At Al-Kut, 160 kilometres south-east of Baghdad, Iraqis prepare free food for the Iranian pilgrims heading to Najaf, the Shia world′s spiritual capital and home to the shrine of Imam Ali

Shia pilgrims from around the world overnight in a roofed outdoor area separating the bazaar in Najaf from the shrine of Imam Ali

A group of Iranians walk the 80 kilometres between Najaf and Karbala to mourn at the shrine of Hussein Ibn Ali on the last day of Arbaeen. In 680 AD, Hussein Ibn Ali was killed in a battle against the caliph of the day. Shia Muslims believe that Hussein was the person who should have been chosen as caliph

On the road between Najaf and Karbala, Iraqi Shia set up thousands of special shelters, called moukeb, to feed pilgrims and provide them with a place to rest and stay over night

Practically every 500 metres, there is an Iraqi family serving Middle Eastern dishes, sweet tea, Arabic coffee and Iraqi dates to pilgrims

As well as food and drink, thousands of banners with religious message are erected along the pilgrims′ route to Karbala. This banner tells the story of Muslim Ibn Aqil, whose sons were killed during the battle of Karbala

Iranian and Iraqi officials also erect banners with political messages. The Arabic sentence on the banner reads ″the Yemeni Karbala″

An estimated 17 million pilgrims from across the world flock to the city of Karbala. During the last days of ceremony, which this year fell in the second week of November, all the streets in Karbala were closed to vehicles

As the black-clad Shia pilgrims enter the city, they face the shrine of Hussein Ibn Ali and recite the specific Arbaeen dua (supplication)

Arbaeen is known as the biggest annual gathering of Shia Muslims worldwide. Since 2014, this commemoration of Ibn Ali′s martyrdom has become a strong symbol of the fight against the extremists of Islamic State