Millions of pilgrims have gathered in Iraq's holy city of Karbala to mark Arbaeen, a sacred Shia Muslim ritual. Arbaeen, also known as Chehlum, commemorates the end of the 40-day mourning period after the killing of Imam Hussein, the Prophet Muhammad's grandson, in a battle in 680 AD. His martyrdom is considered a defining event in the schism between Sunni and Shia Muslims.

Iraqi state-run media said more than 22 million pilgrims have visited Karbala, setting a record for the annual pilgrimage, though that figure could not be independently verified. Last year's Arbaeen drew 17 million pilgrims according to official sources. Arbaeen is the world's largest annual gathering, with the number of pilgrims far exceeding the two million visitors who descend on Mecca for the Hajj. The Kumbh Mela, a mass Hindu pilgrimage, attracts more pilgrims but is held only every three years.

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Shia Muslim pilgrims gather in Karbala for the Arbaeen religious festivalMohammed Sawaf/AFP
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An aerial view shows the shrines of Imam Abbas ibn Ali (front) and Imam Hussein during the Arbaeen religious festivalMohammed Sawaf/AFP
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An aerial view shows pilgrims gathering at the shrine of Imam Abbas ibn Ali, brother of Imam Hussein, in the holy city of KarbalaMohammed Sawaf/AFP
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Pilgrims reach out to touch the tomb of Imam al-Abbas located inside the Imam al-Abbas shrineAhmed al-Husseini/Reuters
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Pilgrims try to touch the tomb inside the Imam al-Abbas shrineAhmed al-Husseini/Reuters
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Shia Muslim pilgrims reach out to touch the tomb of Imam al-AbbasAhmed al-Husseini/Reuters
Iraq: Army brought in as millions of Shi'ite Muslims descend on Kerbala for ArbaeenIBTimes UK

Many made the pilgrimage on foot from Baghdad, a journey of 80km which can take days. Some walked from as far as Basra in southern Iraq, a journey of nearly 700km. Hundreds of thousands of pilgrims came from neighbouring Iran; a journey that takes about a month on foot. Although Arbaeen is a Shia ritual, some Sunni Muslims and people from other religions such as Christianity and Zoroastrianism also made the pilgrimage.

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Pilgrims from Basra lead their camels as they walk through Najaf towards KarbalaHaidar Hamdani/AFP
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Pilgrims from Basra walk through Najaf on their way to the holy city of KarbalaHaidar Hamdani/AFP
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Shia Muslim pilgrims walk to the holy city of Kerbala, ahead of ArbaeenAlaa al-Marjani/Reuters
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Pilgrims walk to the holy city of Kerbala, for the Arbaeen ritualAlaa al-Marjani/Reuters
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A woman touches a portrait of Imam Hussein in the city of NajafHaidar Hamdani/AFP
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Grand Ayatollah Sheikh Bashir Hussain Najafi, one of the five Grand Ayatollahs in Iraq, walks through Najaf on his way to KarbalaHaidar Hamdani/AFP

A special task force of Shia militiamen was deployed to provide extra security in Karbala and along the route of the pilgrimage. Iranian advisers are also helping to coordinate security. The processions of marchers have previously been a target for Sunni Islamist insurgents. Islamic State targets Iraq's Shia Muslims, viewing them as apostates deserving of death.

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A member of the Iraqi presidential guard stands guard in Baghdad's Dora district as pilgrims head on foot towards the holy city of KarbalaSabah Arar/AFP
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Security staff search pilgrims at the entrance to KarbalaMohammed Sawaf/AFP
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The shadow of an Iraqi security forces helicopter is seen near a truck transporting pilgrims in KarbalaMohammed Sawaf/AFP

Pilgrims dressed in black gathered outside the golden-domed Imam Hussein shrine in an act of grief and mourning. Hussein's death entrenched the schism between Shia and Sunni Islam, which has sharply divided Iraq for years. The sectarian conflict gripping Iraq, mostly between the Sunni minority and the now dominant Shia, has been exacerbated by the rise of the ultra-hardline Sunni militant group IS (Daesh).