The Hajj chant of "Here I am, oh Almighty" reverberated through the air as the pilgrims braved sweltering temperatures to make the climb.
They are spending the day praying for forgiveness, before descending to begin the three-day festival of Eid al-Adha at the end of the pilgrimage, which is one of the five "pillars", or duties, of Islam and, as such, is required to be made at least once in a lifetime by every able-bodied Muslim who can afford it.
Police helicopters were hovering above to monitor the slow movement of people.
The Saudi authorities are reported to have spent $200m this year as part of a long-term plan to improve safety.
No incidents have been reported so far.
New facilities
The pilgrims spent the night in huge tent camps on the plains of Mina about 12km (eight miles) from Mecca.
Many of the faithful - the men dressed on a two-piece seamless white robe and the women covered except for their hands and face - carried umbrellas to keep off the desert sun.
Thousands of sprinklers sprayed a fine mist to cool off the pilgrims, many of whom faint during the slow climb.
Volunteers were handing out chilled water and fruit juices, provided free of charge by the Saudi Government.
In addition to mobilising an army of doctors, policemen and municipal employees, the authorities have erected 44,000 air-conditioned and fire-proof tents to house the pilgrims.
Among the hajjis is this year's youngest pilgrim, a newborn Egyptian girl, whose mother went into labour on Friday.
Political messages
"Oh, may God unify the Islamic nation," prayed 45-year old Fuad Issat, an Egyptian, between his prayers.
Correspondents say the Middle East crisis appeared not to be too far from every pilgrim's lips.
A Palestinian from the Gaza Strip called on God to "put an end to Israel's occupation" of the West Bank and Gaza.
He spoke of Jerusalem which was in the hands of "raping Jews who are bullying our Muslim brothers in Palestine," before blasting "these people who speak of rights, whose mouths spread venom on Islam".
"Some people speak of human rights and choose to forget the rights of our brothers who have been killed, expelled and had their lands violated," the cleric said.
On Monday, the pilgrims will throw stones at pillars that symbolise the devil before joining the rest of the world's Muslims in celebrating the Eid al-Adha feast.