The pilgrimage rites known as Hajj, trace the journey made by Prophet Mohammed more than 1,400 years ago.
Draped in white robes, they came on foot, in buses, on motorbikes and even in wheelchairs, from Mecca to Mina some five kilometres to the east.
The pilgrims will spend the night in Mina in prayers and meditation, before heading further south at dawn to Mount Arafat for the climax of Hajj.
Overcrowding at Mina forced thousands of pilgrims to camp out in portable tents on the median of busy roads, under bridges or on the foothills of the arid valley.
Mohammed al-Hakami, 27, sits on the median outside a blue and red nylon tent, as his wearing 90 year old father sleeps on a cardboard box.
Mr Hakami has come from the southern Saudi province of Jizan, six hours away with father, who is diabetic and suffers from high blood pressure.
"I am afraid for him but he insisted on coming," said Hakami.
Long Distances
Amna Mohammed Hassan, 40, left her six children with her mother in central Sudan and came for the Hajj with her husband, brother-in-law and four other elderly female relatives.
They took a 14-hour boat journey to the port city of Jeddah, west of Mecca.
Sitting in a circle on a curbside listening to a sermon by one of their companions, Ayub Khalafallah, 27 said, "Islam is in crisis, we must get our house in order and not only blame the West or our governments."
Religious Obligation
The Hajj is one of the five pillars of Islam and a once-in-a-lifetime obligation for all devout, able-bodied Muslims with the means for the journey.
Its rituals begin with the circling of the Kaaba, the huge black cube in the centre of Mecca's Grand Mosque, which Muslims around the world face when they offer their prayers five times daily.
The faithful believe the Almighty is especially close and receptive in the House of God.
Many pilgrims come despite their poor health and brave the perils of Hajj which, in the past, has included deadly stampedes and campfires.
Some pilgrims even cherish the opportunity to die during their pilgrimage, perceiving it as an opportunity for martyrdom - an idea assailed by many clerics.
Saudi’s King Abdullah, members of the royal family, parts of the religious establishment and several ministers will come to Mecca to be close to the progression of Hajj, which is a matter of national pride for the ultra-conservative kingdom.
The country considers itself a beacon for the world's estimated 1.3 billion Muslims.
High Alert
Saudi Arabia remains on high alert, to prevent another tragedy after a deadly hotel collapse.
The Hajj started three days after an aging hostel in the heart of Mecca collapsed, killing 76 people and wounding 62.
Saudi authorities called off rescue operations Friday and the rubble of the Luluat al-Kheir (Pearl of Grace) hostel was removed to allow the pilgrims to flow to Mina.
Even some of the wounded left their hospital beds and went to Mina in specially- equipped ambulances that will transport them to Mount Arafat.
The sheer number of people creates havoc for security officers.
"Get off the street please," pleaded security officers through loudspeakers as they drove up and down Mina's main roads.
Their task looked next to impossible as some oblivious pilgrims prayed; others ate ice cream, as the smell of barbecued kebab filled the air.
"We cannot remove them by force, there is simply not enough room in the tents for everyone," said one Saudi security officer.
The most dangerous part of the Hajj takes place on Tuesday when pilgrims come back to Mina from Arafat to stone three pillars symbolising the powers of the devil.
The area has been the scene of several deadly stampedes in the past.
To prevent similar occurrences, the Saudis have transformed the three pillars from tall obelisks to stone walls, allowing many more people to pelt them with pebbles at once.
A religious fatwa also extended the hours permitted for the ritual.
Balloons and national flags fluttered above the sea of white tents housing pilgrims who came as part of organised tours registered with the ministry of Hajj.
Almost 60,000 security, health, emergency and other personnel are involved in organising the Hajj this year, trying to make sure none of the deadly incidents that have marred it in the past are repeated.
In addition to stampedes that killed 251 pilgrims in 2003 and 1,426 in 1990 and a camp fire in 1997 in which 343 perished, authorities have to tackle the threat of disease, rioting and terrorist attacks.
More than 2,000 pilgrims have died at the Hajj since 1987.
As part of the security measures, Saudi television is repeatedly broadcasting instructions to the pilgrims to avoid forming into crowds, and not to camp along the pathway.
And Authorities have widened walkways this year to reduce congestion.
Bird Flu
The health ministry said it was not worried about avian flu, which has killed three children in Turkey over the past week.
"All the pilgrims that already came in are disease free," said spokesman Khaled al-Mirghalani.
Pilgrims were required to show proof they were vaccinated for common contagious diseases before being allowed into the country.
Nine thousand six hundred doctors, medics and other staff were on standby at 14 hospitals, clinics and field facilities to deal with any contingency.
"I feel relaxed that we are ready, the rest is in God's hands," said Tareq al-Arnous, the head of emergency operations at the ministry.
A similar attitude was echoed by some of the pilgrims leaving for Mina.
"Whatever God ordained will happen," said Ahmed Omar, 40, from Somalia, accompanied by his wife and his wheelchair-bound mother-in-law.
Another pilgrim from Yemen said he was worried about potential accidents after the hotel collapse.
"But I must do this for God," said 50 year old Qaed Mehdi.
Zamzam or Holy Water
At the Hajj, pilgrims are expected to drink more than 1,641 cubic litres of water everyday from a 35-metre-deep well, which is located within the Holy Mosque.
Known as Zamzam water it is taken from the sacred well of Zamzam and is thought to cure the sick.
“God said He would grant the person who drinks Zamzam water his wish. If you drink this water with good intentions, with the aim of pleasing God, and to cure yourself of a disease, or to achieve a certain goal, God will grant you that wish,” said, pilgrim Mohammed Issat from Egypt
According to Arab historians, the Zamzam Well, except for a few periods when it became dry or was buried under sand, has been in use for around 4000 years.
It marks the site where the Prophet Abraham, under God's command, had left his wife Hajar and their infant son Ismail.
In her desperate search for water, Hajar ran seven times back and forth across the desert to find water for Ismail, who was dying of thirst.
It is believed that God sent the angel Gabriel, who scraped the ground, causing the spring to appear.
The name Zamzam originates from the phrase Zom Zom, meaning 'stop flowing', a command said to have been repeated by Hajar during her attempt to contain the spring.
The area around the spring, which was later converted to a well, became a resting place for caravans, and eventually grew into the trading city of Mecca, the birthplace of the Prophet Mohammed.
Prophet Abraham later returned there to rebuild the first House of God.
Because of its square shape, it is called The Kaaba and is the holiest Muslim shrine.
The Kaaba now stands in the centre of the Holy Mosque, also called Al-Haram al-Sharif.
And according to tradition, the Prophet Mohammed is quoted as saying that if the sick drink the water and pray to God for help then they will be healed.
Saudi Health Minister, Hamad Maneh, says that the water is monitored by the authorities to make sure it is fit for drinking.
"An office is responsible for the Zamzam water and makes sure that it is clean and suitable for drinking.”
The water that is given to people to drink is thoroughly checked so that it doesn't contain any microbes and so hopefully it is suitable for drinking and will not cause the transmission of any diseases," he added.