Saudi has 100,000 security people on alert in case of attacks, deadly stampedes.
Saudi Arabia has mobilised a force of 100,000 men to protect up to three million Muslim pilgrims starting the annual Haj on Saturday, amid fears of attack or deadly stampedes.
All pilgrims from abroad, totalling 1,728,841, have entered the kingdom, the highest recorded number coming from outside Saudi Arabia for the ceremony, the interior ministry said on Saturday.
The majority - over 1.57 million - arrived by air, Saudi Interior Minister Prince Nayef bin Abdul Aziz was quoted by SPA official news agency as saying, adding that pilgrims came from 178 nationalities.
Several hundred thousand Saudis and other people resident in the country usually join visiting pilgrims in the annual rites, which last several days.
Local press reports spoke of a throng that could reach three million and SPA news agency reported than everything was proceeding smoothly, with no threat yet of any stampedes like those which caused many fatalities in some previous years.
The Haj is one of the five pillars of Islam which the Koran says all Muslims must carry out at least once in their lives, if they are able to do so.
It began early on Saturday as pilgrims arrived on foot or by bus in Mina, a dusty valley 10km from the holy city of Mecca.
Dozens of first aid stations lined the route, SPA reported.
Pilgrims are spending 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 nearby Mount Arafat. Their gathering at the hill and its surroundings symbolises the wait for the last judgment and is the high point of the Haj.
Next, the faithful return to Mina to sacrifice an animal, usually a sheep, to commemorate the willingness of Abraham to sacrifice his son on God's orders. This marks the start of the Eid Al-Adha, the feast of the sacrifice.
They will spend another two days in Mina for the final rite, the stoning of Satan. Each pilgrim throws 21 pebbles at each of three pillars symbolising the devil.
This year for the first time participants will receive the necessary pebbles in pre-packed bags to spare them the effort of searching for the stones.
A special Haj committee organised the collection, washing and packaging of the pebbles after first obtaining a fatwa, or religious edict, permitting the initiative, the Al-Watan daily said.
The stoning is the most hazardous of the rituals. The pillars were enlarged a few years ago into 25m high concrete blocks. Bridges have been built at three levels at the site to help prevent a recurrence of fatal stampedes.
The vast tide of humanity massing in relatively small spaces has been the source of the Haj's bloodiest disasters, with stampedes causing the deaths of 364 people in 2006, 251 in 2004 and 1,426 in 1990.
The Saudi health ministry said it has put in place some 24 field hospitals with 4,000 beds, in addition to nearly 140 health centres. They will be served by more than 11,000 medical staff and a fleet of 150 ambulances.
After the stoning ceremony, the pilgrims go to Mecca'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.
US-made Sikorsky S-92 helicopters fitted with sophisticated technology such as night vision equipment are being used for the first time amid the spectre of the attacks which have haunted Saudi Arabia in recent years.
"[Saudi forces] are ready to cope with their responsibilities," Prince Nayef had said after inspecting the security forces supervising the Haj.
"Terrorism is not finished. It is still going," Prince Nayef told journalists ahead of the pilgrimage.