Saudi officials predicted an estimated 1.5 million pilgrims would converge on the site.
That number is down sharply from last year's 3.2 million after Saudi authorities slashed hajj quotas because of massive construction works at the Mecca Grand Mosque.
Fears linked to the Middle East Respiratory Syndrome (MERS) virus were another reason behind the reduced number of pilgrims. The disease has killed some 60 people worldwide, most of them in Saudi Arabia.
The hajj is one of the five pillars of Islam that every capable Muslim must perform.
This year's hajj will officially end on October 18.
Saudi Arabia has deployed more than 100,000 troops to ensure the safety of the pilgrims.