The hajj by the numbers

In Mecca, pilgrims pray while they walk around the cube-shaped Kaaba at the center of one of Islam's holiest sites. All Muslims who are physically and financially able are expected to visit the holy site at least once in their lifetime. More than 2 million pilgrims are expected to attend the hajj this year; in 1941 there were just 24,000 attendees.

Walking counter-clockwise, Muslims circle the cube-shaped Kaaba, which represents the metaphorical house of God, as they recite prayers.

Some 54 million pilgrims have attended the hajj over the past 25 years. Saudi officials believe that, by 2030, some 30 million pilgrims will visit the country's holy sites at Mecca and Medina every year.

Officials say 8 million copies of the Quran, in various translations, as well as other religious books will be handed out during the five-day gathering.

The threat of a terror attack remains ever present but stampedes have historically posed the biggest threat. Thousands have been trampled to death over the years. The deadliest stampede occurred in 2015 when more than 2,000 people were killed, according to three international news agencies. The Saudi government maintains the death toll was 769.

Some 25 hospitals along with 180 ambulances and 30,000 health particioners are on hand to provide emrgency medical care. Tens of thosuands of air-conditioned tents have also been set-up as temperatures are expected to top 40C (105F) throughout the week.

About 16,000 telecommunication towers have been installed, along with 3,000 Wi-Fi access points.

Some 18,000 civil defense employees are among the tens of thousands of security personnel. Thousands of security cameras have also been deployed.

Pilgrims are arriving by land and by air. About 14,000 international and domestic flights, along with 21,000 buses, have ferried pilgrims to the holy site.