MINA, Saudi Arabia — The world's largest annual pilgrimage, the hajj, began on Sunday with hundreds of thousands of Muslims pouring into the camp of Mina from Mecca to prepare for the solemn rituals.

This year's number of pilgrims is estimated at up to 2.5 million, posing a major headache for the Saudi authorities who, however, have yet to report any major incidents since the faithful descended on the holy city.

On Sunday afternoon, pilgrims were still flooding into the vast plain of Mina, a small village about three miles east of Mecca, using all possible means to begin their hajj journey.

Buses, choked with both people and luggage inside, carried yet more on their roofs. Tens of thousands sat on the pavements, many of them with their tents for camping.

Their sirens screeching, police cars threaded their way into the crowds in an attempt to keep roads open.

Many pilgrims took the buses, but others had set off on foot overnight as they headed for the village that comes to life for just five days a year.

Authorities say permits have been granted to 1.7 million foreign pilgrims, with a further 200,000 or so issued to pilgrims from within Saudi Arabia and from neighboring Gulf states.

An interior ministry official said definite numbers will not be announced until Tuesday, the first day of Eid Al-Adha or the Feast of the Sacrifice.

This year has seen a crackdown on pilgrims who do not have the requisite papers as the authorities try to prevent numbers from getting out of hand.

A driver caught transporting unauthorized pilgrims faces a fine of 10,000 riyals (2,667 dollars) for each one. Vehicles with a capacity of fewer than 25 passengers have also been banned from entering hajj sites to streamline the flow of buses.

The passage to Mina marks the official launch of the hajj on the eighth day of the Muslim calendar month of Dhul Hijja.

The day is known as Tarwiah (Watering) as pilgrims in the past stopped at Mina to water their animals and stock up for the next day's trip to Mount Arafat.

At Mount Arafat, some six miles southeast of Mina, the pilgrims spend the day in prayer and reflection.

After sunset, they move on to Muzdalifah, half way between Mount Arafat and Mina, where they spend the night.

On Tuesday, the pilgrims head back to Mina after dawn prayers.

They then perform the first stage of the symbolic "stoning of the devil" and make the ritual sacrifice of an animal, usually a lamb.

On the remaining three days of the hajj, the pilgrims continue the ritual stoning before performing the circumambulation of the Kaaba shrine in Mecca and then heading home.

This year has been incident-free since the pilgrims began gathering in Mecca. The city's Grand Mosque has been flooded with the faithful, with an estimated 1.7 million taking part in the main weekly Muslim prayers on Friday.

The movement of pilgrims between the holy sites is a major worry for the authorities who have had to deal with deadly stampedes in the past.

Saudi Arabia has used its huge oil revenues for massive spending on new infrastructure to ease the flow of humanity.

This year, the first phase of the new Mashair Railway -- or Mecca metro -- will transport pilgrims between Mina and Mount Arafat through Muzdalifah.

The Jamarat Bridge, where the ritual stoning takes place, has also been expanded to five levels with movement channelled in one direction.

Security also remains a concern. Last Wednesday, Interior Minister Prince Nayef bin Abdul Aziz said he could not rule out the possibility of a sabotage attempt by Al-Qaeda.

King Abdullah on Saturday appointed Prince Nayef, his second deputy prime minister, to replace him overseeing the hajj as he is resting because of a herniated disc.