Muslims from all over the world started one of Islam's most sacred rituals in Mecca, Saudi Arabia Tuesday. 

The rite is required of able-bodied faithful who can afford it at least once in a lifetime to cleanse the soul and wipe away sin.

It is one of the largest annual migrations and coincides with a period during the 12th month of the Muslim lunar calendar.

The hajj -- a spiritual journey of prayers and rituals -- comes to a climax yesterday.

This year some 3 million people are expected at the birthplace of the prophet Mohammed.

Hajj Minister Iyad Madani said more than 1.5 million pilgrims from 160 countries and regions have arrived in Saudi Arabia, and would be joined by another 500,000 from within the kingdom. Up to 1 million residents of Mecca, the holiest city in Islam, could also take part, he added.

The millions of pilgrims often strain the municipal infrastructures in the holy cities.

On Monday, they crammed into the Grand Mosque to walk around the Kaaba, an ancient cube-shaped structure covered in a black cloth and housing a sacred black stone from which Mohammed is said to have started his own pilgrimage.

Defying unseasonably hot weather, the influx of pilgrims transformed the center of Mecca into a kaleidoscope of white robes worn by men and women's colorful costumes.

Officials provided water and emergency treatment to pilgrims fainting in the crush as they walked around the Kaaba performing ritual chants, while many surged for the Kaaba itself.

Some found time to receive mobile phone calls, as the majority tried to make their way round seven times softly muttering prayers and reading Koranic verses.

Late on Monday they began streaming from Mecca into the tent city of Mina in preparation for a day of prayer and meditation.

The white fireproof tents will shelter pilgrims during the night before they continue their journey today to Mount Arafat for the zenith of the rituals that symbolize the Final Day of Judgment.

They will return to Mina tomorrow after spending the night in the town of Muzdalifa, to stone the Jamrat or three pillars representing the devil.

Once back in Mina, pilgrims will sacrifice animals to mark the beginning of one of Islam's most important holidays: Eid al Ahda or the feast of sacrifice commemorating God's provision of a ram for Abraham to sacrifice as he was about to slay his son to show his devotion.

More than a million sheep have been imported, mainly from Sudan and Uruguay.

Pilgrims will spend two more days in Mina taking part in the symbolic three-day stoning of the devil.

Saudi authorities have mobilized more than 50,000 security personnel to ensure a peaceful hajj. More than 10,000 officers have been deployed to deal with security related developments.

Interior Minister Prince Naif, who toured the Holy Sites in Mecca on Saturday to inspect the hajj preparations and new projects, said the kingdom had made sufficient security arrangements. He added that Saudi security forces are fully prepared to meet any contingency.

Extra police have been posted at the entrances to Mecca and on roads leading to the Holy Sites. More than 14,200 buses will be used to transport pilgrims.

The Health Ministry is on the alert also. The hajj Health Committee chaired by Dr. Adnan al-Bar, director of health in Mecca, said all health centers and hospitals in Mecca, Mina, Arafat and Muzdalifa were well-equipped and ready.

"We have huge stockpiles of medicines to combat any outbreak of disease. There are so far no reports of contagious diseases among the pilgrims," he said.

More than 12,000 food outlets have been set up to feed the faithful. Bakeries are equipped to make 10 million loaves of bread every day.

The preparations are the result of tragic experience.

Deaths are not uncommon during the pilgrimage, often resulting from stampedes that have a domino effect on the mass of humanity that descends on the area.

Last year, a rush of people left 244 dead -- including five Chinese. Another 14 Chinese were injured.

Already this year there have been some incidents.

A fire broke out Monday in a neighborhood close to the Grand Mosque as thousands of pilgrims converged on the Muslim holy site for prayers.

Saudi officials said initial reports indicated the blaze caused no injuries and was quickly extinguished. The fire was in an apartment building in the Gazza neighborhood and it was extinguished in 10 minutes, a Saudi security official said.

Gazza is about 2 kilometers from the Grand Mosque, which houses the sacred black cubic structure known as the Kaaba that Muslims around the world face in prayer five times a day.

The blaze did not interrupt the thousands of pilgrims who converged on the mosque and other nearby holy sites.

Turnout was so high that worshippers were forced to stand and pray instead of the traditional bowing on a carpet.