Muslim pilgrims gather at Mount of Mercy on the plain of Arafat during the annual haj pilgrimage, outside the holy city of Mecca, Saudi Arabia (Reuters Photo)
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The Hajj, one of the world's biggest religious gatherings, is increasingly affected by climate change, according to a Saudi study.(Reuters Photo)
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Saudi authorities have urged pilgrims to drink plenty of water and protect themselves from the sun. (Reuters Photo)
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The Hajj is one of the five pillars of Islam and all Muslims with the means must perform it at least once. (Reuters Photo)
After Mount Arafat, the pilgrims will head to Muzdalifah, where they will collect pebbles to carry out the symbolic "stoning of the devil" ritual in Mina on June 16. (Reuters Photo)
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The Hajj is said to follow the path of the Prophet Mohammed's final pilgrimage, about 1,400 years ago. (Reuters Photo)