It is one of the greatest examples of human devotion on Earth - a sea of white robes as hundreds of thousands of Muslims perform the evening prayer at the Grand Mosque in Mecca.
The pilgrims, including Iranian president Mahmoud Ahmadinejad, started the week-long religious event at the Saudi Arabian city with a three-mile walk to the holy site of Mina.
They have travelled to the holy city in preparation for the haj, the largest annual pilgrimage in the world.
Pilgrims make the journey to Mecca in the second week of Dhual-Hijjah - the twelfth month in the Islamic calendar - and perform prayers and rituals in the days before the haj begins.
It is the duty of every able-bodied Muslim to make haj at least once in their lifetime.
In a display of solidarity and faith, the pilgrims walk seven times around the Kaaba - the cube-shaped structure at the centre of the picture - touching the sacred black stone at its corner.
They wear sacred robes called ihram and are forbidden to use cosmetics, shave or cut their nails while dressed in these. They are also not allowed to swear, argue, kill a living creature or have sex.
Over the five days of haj, more than two million Muslims, 25,000 of whom are British, will pass through the Grand Mosque.
The vast number of people and atmosphere of religious fervour has meant that techniques in crowd control have become increasingly important.
Previous years have seen stampedes - in January 2006, 346 Muslims were crushed and a further 200 were injured on an overcrowded bridge. This year, more than 35,000 security staff are on hand to ensure that this show of unity is a peaceful one.