More than two million Muslim pilgrims from all over the world gathered for the annual Hajj pilgrimage in Mecca, Saudi Arabia. A sprawling tent city, stretching as far as the eye can see, was set up to house the pilgrims.

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An aerial view of the tent city set up for Muslim pilgrims in Mina, near the holy city of Mecca, Saudi ArabiaReuters
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Muslim pilgrims pray around the holy Kaaba at the Grand Mosque in MeccaReuters
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Muslim pilgrims holding umbrellas to shelter from the heat walk on a bridge as they head to Mina to cast stones at pillars symbolising SatanReuters

The pilgrimage is physically demanding and involves performing several rites, such as circling the cube-shaped Kaaba seven times at the start and finish of the Hajj. It is a main pillar of Islam and one that all able-bodied Muslims must perform once in their lives.

The five-day spiritual journey is meant to cleanse the faithful of sin and bring them closer to God. All male pilgrims dress in simple, white robes as a sign of equality before God.

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Pilgrims walk counter-clockwise around the Kaaba while others stop to touch itReuters
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A man kisses the wall of the Kaaba in the Grand MosqueReuters
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The Abraj Al-Bait Tower, also known as the Mecca Royal Hotel Clock Tower, towers over the Grand Mosque as Muslim pilgrims circle the KaabaReuters
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Pilgrims worship in the Grand Mosque, during the annual Hajj pilgrimageReuters

Pilgrims gathered on a desert hill near Mecca in an act of faith and repentance during the climactic emotional and spiritual moment of the annual Hajj pilgrimage.

Men and women wept openly at Mount Arafat as they stretched their hands out in prayer and supplication, saying "Labayk, Allahuma, labayk," — "Here I am, God, answering your call. Here I am."

The faithful believe that on this day the gates of heaven are open, prayers are answered and past sins can be forgiven.

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Muslim pilgrims gather on Mount Arafat, near MeccaAFP
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A Muslim pilgrim prays on Mount Arafat, near Mecca, as he takes part in one of the Hajj ritualsAFP
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Muslim pilgrims perform Friday prayers around Namirah mosque on the plains of ArafatReuters
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Pilgrims are sprayed with water to cool them down as they leave the plains of ArafatReuters

In Mina, a desert tent city just outside the Saudi holy city of Mecca, pilgrims cast pebbles in a symbolic stoning of the devil. Male pilgrims changed out of their white pilgrim robes and shaved their heads as a sign of renewal. Women clipped a lock of hair.

Though pilgrims will repeat the stoning ritual for two more days, they can now be referred to as "Hajjis," a term of honour for completing the pilgrimage.

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Muslim pilgrims throw pebbles at pillars symbolising Satan in Mina, near the holy city of MeccaReuters
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Muslim pilgrims cast stones at pillars symbolising SatanReuters
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One of the pillars symbolising Satan is seen during the annual Hajj pilgrimageReuters
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Huge crowds of Muslim pilgrims arrive in Mina, near Mecca, to cast stones at pillars symbolising Satan, during the annual Hajj pilgrimageReuters
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A man has his head shaved after throwing pebbles at pillars during the ritual stoning of SatanAFP
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A Muslim pilgrim has his son's head shaved after casting seven stones at a pillar that symbolises Satan during the annual Hajj pilgrimage in Mina, near MeccaReuters
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Muslim pilgrims pray after they cast stones at pillars symbolising Satan, during the annual Haj pilgrimage on Eid al-Adha in Mina, near the holy city of MeccaReuters

Major General Mansour Al-Turki, spokesman for the Saudi Interior Ministry, said the kingdom is facing continuous threats from terrorists, but is prepared to ensure a safe Hajj.

Saudi Arabia and four other Arab countries are taking part in US-led air strikes against the Islamic State group and al-Qaida fighters in Iraq and Syria. Militants have vowed revenge.

The routes for Hajj pilgrims and inside the Grand Mosque housing the Kaaba have thousands of security cameras, many of them hidden. The kingdom says there are some 70,000 security personnel guarding the Hajj this year.

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Saudi policemen watch screens showing footage from cameras set up around Hajj sitesReuters
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Security officials patrol a checkpoint between Jeddah and Mecca. The Saudi Interior Ministry prohibits non-Muslims and those without permission for the pilgrimage from entering the holy city of MeccaReuters