Up to two million Muslims from around the world have converged on Mount Arafat in Saudi Arabia for the climax of the annual Hajj pilgrimage.
The worshippers, dressed alike in white seamless garments, performed a series of rituals on the hill 18 kilometres (11 miles) west of Mecca.
The Hajj is one of the five pillars of Islam, which every able-bodied Muslim is expected to perform at least once.
Saudi Arabia has increased security after last year's bombings in Riyadh.
More than 25,000 police and other security personnel have been deployed around Mecca.
The pilgrims - dressed in traditional white robes - gathered on Mount Arafat, where the Prophet Mohammad is said to have given his last sermon 14 centuries ago.
Pilgrim Habib Miyan: Possibly the oldest man on Earth
At a special sermon at Namirah mosque in Arafat, Saudi Arabia's top cleric condemned Muslim militants who carry out suicide bombings.
"Is it holy war to shed Muslim blood? Is it holy war to shed the blood of non-Muslims given sanctuary in Muslim lands?" Sheikh Abdulaziz al-Sheikh told worshippers.
The BBC's John Simpson in Jeddah says the authorities' mustering of so many pilgrims over a period of just five days is an extraordinary feat of organisation.
They have provided 44,000 fireproof and air-conditioned tents, with water supplies and cooking facilities, in the Valley of Mina, about four kilometres (2.5 miles) from Mecca.
Benefactor
The pilgrims include Habib Miyan, an Indian man aged 125 according to his pension papers, although he claims to be older.
His pilgrimage was funded by a Yorkshire taxi driver and two businessmen - one British, the other American.
Mr Miyan, who is generally healthy but has been blind for 50 years, said he would pray for his benefactors.
Prayers for guidance and forgiveness on Mount Arafat are regarded as the culminating experience of a Muslim's life and will be followed over the next few days by the mass slaughter of animals for Eid al-Adha, the Feast of the Sacrifice.
In Prophet's footsteps
Before dawn on Saturday, the faithful were moving towards Mount Arafat.
"
It's the greatest gathering of mankind anywhere
"
Zulfiqar Ali, Lancashire, UK
The time spent here is believed to symbolise Judgement Day, when Islam says every person will stand before God and answer for his deeds.
In the months since the suicide bombings in Riyadh, there have been a series of clashes between security forces and Islamic militants.
Vast quantities of explosives and weapons have been seized in raids throughout the kingdom.
On Thursday, five members of the security forces were killed in a gun battle with militants in the capital Riyadh.
Last November, the government said it had arrested several al-Qaeda militants in Mecca who were planning to attack pilgrims.
Dr Ahmad Turkistani, of Al-Imam University in Riyadh, said conferences were being held to coincide with the festival to discuss how to improve relations between the West and the Muslim world.
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