Muslims save for years to perform the Hajj

The ascent to Mount Arafat, a hill 18km (11 miles) west of Mecca, is the high point of the five-day rituals.

Saudi officials have mobilised more than 50,000 security staff to prevent possible stampedes or terror attacks.

All able-bodied Muslims who can afford to must perform Hajj at least once in their lifetime.

Pilgrims pray

From sunrise, convoys of more than 20,000 buses carried the pilgrims to Mount Arafat, where the Prophet Muhammad is believed to have given his last sermon 14 centuries ago.

Tens of thousands gathered for prayers in the Namera mosque on Arafat, a small plain surrounded by mountains.

Speaking from the mosque, Sheikh Abdul Aziz al-Sheikh, the kingdom's highest religious authority, said the greatest challenge for the Muslim nation came from offspring who had been led astray.

"The greatest affliction to strike the nation of Islam came from some of its own sons, who were lured by the devil," he said.

He warned of increasing hostility towards Islam and spoke of "military campaigns, thought campaigns, economic campaigns and media campaigns" against the religion.

Hajj: Your experiences In pictures: Hajj begins

Thousands of the faithful - men clad in a seamless white cloth and women with their faces uncovered - prayed for forgiveness and asked God for help.

"I am delighted to be here," Mohammed Tahrio, a 28-year-old Nigerian, told the AFP news agency. "I want to pray to Allah to give me a long and wealthy life."

Amer Abbas, 45, from Diyala in Iraq, said he would be praying to God "to make the Americans leave our country and put an end to the occupation".

Later, the pilgrims gathered small pebbles to use the next day, when they are to return to their tent city in Mina and throw stones at pillars called Jamaraat, which represent the devil.

Relieve crowding

The authorities in Saudi Arabia have brought in a range of measures designed to ensure the wellbeing and safety of the pilgrims.

Saudi Arabia has brought in new measures to ensure pilgrims' safety

Three makeshift hospitals and 46 clinics were set up near Arafat to give medical assistance if required, while charity workers gave out food and drink.

Security has been stepped up in response to fears of possible attacks, with counter-terror squads in operation and 50,000 police and soldiers deployed.

Helicopters hovered above the plain all the way to the peak of Mount Arafat.

The authorities are also keen to prevent a repeat of the stampede during last year's stone-throwing ritual in which 251 people were trampled to death.

New pillars put up for the stoning of the Jamaraat are wider and taller so more pilgrims can throw rocks at the same time, alleviating crowding.

STAGES OF THE PILGRIMAGE BY DAY The Hajj takes place over five days