More than two million Muslims have taken part in a prayer ceremony on Mount Arafat - one of the main events in the annual pilgrimage to Mecca.
New security measures are in place for the Hajj
The pilgrims will also hear a sermon modelled on the one the Prophet Muhammad is believed to have given at the site about 1,400 years ago.
The ritual forms the spiritual climax of the Hajj.
New safety measures have been added to the Hajj, aimed at preventing stampedes that have killed hundreds previously.
In January, almost 400 people were killed and some 300 injured in a stampede during one of the rituals.
The Hajj, one of the five pillars of Islam, is due to end on Monday.
It is an obligation for all Muslims to undertake the pilgrimage at least once in a lifetime, if they are physically and financially able.
The Prophet Muhammad is believed to have given his last sermon at the site of Mount Arafat.
One of the major themes of that sermon was equality between men and women and rich and poor.
The pilgrims are clad in identical white robes in scenes that are meant to symbolise the day of judgement.
At the end of the day, thought to be one of the most important in any Muslim's life, pilgrims believe their sins will be absolved.
On Thursday morning, pilgrims from more than 70 countries made the short journey from Mecca to the tented city of Mina five km (three miles) away, which will be their home for the next four days, and from where they perform the rituals of Hajj, marking the re-enactment of the trials of Abraham.
2006: 345 die in a crush during a stone-throwing ritual
2004: 251 trampled to death in stampede
2003: 14 are crushed to death
2001: 35 die in stampede
1998: At least 118 trampled to death
1997: 343 pilgrims die and 1,500 injured in fire
1994: 270 killed in stampede
1990: 1,426 pilgrims killed in tunnel leading to holy sites
1987: 400 die as Saudi authorities confront pro-Iranian demonstration
Before the pilgrims complete the Hajj they must walk seven times round the Kaaba (a cube-like building in the centre of the city's Great Mosque) in an anti-clockwise direction.
The Saudi authorities are imposing a strict quota system to try to keep the number of foreign visitors to a manageable level.
At the last Hajj, earlier this year, at least 345 pilgrims died in a crush during the stone-throwing ritual of the pilgrimage.
The stampede took place at the foot of the bridge of Jamarat, in Mina, where pilgrims hurl stones at three pillars representing the spot where the devil is said to have appeared to Abraham, and which creates a dangerous bottleneck.
Since then a major rebuilding project has been undertaken and 50,000 security personnel have been mobilised.
The ritual has seen many lethal stampedes, but the number of dead in January was the highest in 16 years.
Pilgrims at the current Hajj are expected to take part in the ritual on Saturday.
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