Saudi Arabia has mobilised a force of 100,000 men to protect an estimated two million Muslim pilgrims flocking to Makkah for hajj, amid fears of attack or deadly stampedes.

For the first time, the authorities have brought in sophisticated US-built helicopters to guard against possible attack during the world's largest pilgrimage, which has often been blighted by tragedy.

Around two million Muslims are expected join the annual ritual, which this year begins on Saturday, including several hundred thousand Saudis and residents of the kingdom and at least 1.6 million from other countries.

"(Saudi forces) are ready to cope with their responsibilities," Interior Minister Prince Nayef Bin Abdel Aziz said after inspecting the security forces, which will supervise the pilgrimage.

US-made Sikorsky S-92 helicopters fitted with sophisticated technology such as night vision binoculars are available to security forces for the first time amid the spectre of attacks, which have haunted Saudi Arabia in recent years.

"Terrorism is not finished. It is still going," Prince Nayef told journalists. Asked about the risk of an attack during the pilgrimage, he said: "We have no information but we must be ready so as not to be caught unawares."

The oil powerhouse has faced a string of attacks against Western targets and oil facilities since May 2003 and hundreds of suspected Islamist extremists or sympathisers have been arrested.

During the 2007 hajj, the Saudi authorities announced the arrest of a group linked to the al-Qaeda network, which planned to carry out an attack, without, they said, targeting Makkah or the pilgrims themselves.

But it is the vast tide of pilgrims massing in relatively small spaces that has been the source of the bloodiest disasters, with stampedes causing the deaths of 364 people in 2006, 251 in 2004 and 1,426 in 1990.

The hajj is one of the five pillars of Islam, which the Quran says all Muslims must carry out at least once in their lives, if they are capable.

It will begin in the early hours of Saturday when pilgrims arrive on foot or by bus in Mina, a dusty valley 10 kilometres (six miles) from the holy city.

The devotees will spend the day in prayer and contemplation in the valley, transformed into a city of fireproof tents.

At dawn on Sunday, they will head for the top of nearby Mount Arafat. Their time at the summit symbolises the wait for the last judgement and is the high point of the hajj.

Next, the pilgrims return to Mina to sacrifice an animal, usually a sheep, to commemorate the willingness of Abraham to sacrifice his son on Allah's orders. This ceremony marks the start of the Eid-ul-Azha (feast of the sacrifice).

They will spend another two days in Mina for the final rite, the stoning pf Satan, which consists of each pilgrim throwing 21 pebbles at each of three pillars symbolising the devil and is the most hazardous of the rituals.

These pillars were enlarged a few years ago into 25-metre (80 foot) high concrete blocks. Bridges have been built at three levels at the stoning site to help prevent a recurrence of the fatal stampedes.

Once the stoning ceremony is over, the pilgrims go to Makkah's Great Mosque for a "farewell visit" to the Kaaba, a cube-shaped structure into which is set the Black Stone, Islam's most sacred relic.