On Friday, around 2 million Islamic pilgrims are expected in Mecca, a holy city in Saudi Arabia, for the start of the annual hajj pilgrimage. Over five days, the faithful follow a path that is said to have been taken by the Islamic Prophet Muhammad around 1,400 years ago. Muslims always try to pray while facing toward the Kaaba — the square shrine seen here — wherever they are in the world.

On Friday, thousands of pilgrims circled the Kaaba counterclockwise seven times. It is Islam's holiest shrine. Pilgrims also tilt their hearts toward the cube-shaped structure as part of the ritual.

The hajj is one of the most diverse gatherings in the world. Several hundred thousands of the more than 2 million pilgrims are from Saudi Arabia. To avoid any risk to safety, Saudi authorities have urged pilgrims not to discuss politics or divisive issues and to focus on worship.

Most making the hajj will spend Friday night at this tent city in Mina, which can house 2.6 million in total. On Saturday, they will leave for Mount Arafat, around 20 kilometers (12.5 miles) east of Mecca. Wealthier hajjis stay in lavish hotels overlooking the Kaaba. But most people stay in simple accommodations; some even sleep on the ground.

For Muslims around the world, the hajj is meant to be a unifying event and pilgrims shed displays of wealth and materialism. Male pilgrims wear simple white clothes. Women go without makeup, nail polish and perfume. The hajj is seen as an opportunity to strengthen faith and absolve sins. All hajjis, regardless of their backgrounds, perform the same rituals.

As one of the five pillars of Islam, all Muslims are expected to make the hajj once in their lifetime, if they have the physical and financial means. Last year, 2.4 million people took part in the hajj, and a similar turnout is expected for 2019. This year, Saudi King Salman invited 200 survivors and relatives of the mosque attacks in Christchurch, New Zealand, that killed 51 people.

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.