More than a million pilgrims joined Islam's most important rite as the Hajj began, with Saudi authorities implementing comprehensive measures to ensure the safety and well-being of all participants.
With temperatures expected to top 40 degrees Celsius (104 degrees Fahrenheit), robed pilgrims slowly circled the Kaaba, the holiest site in Islam at the heart of Mecca's Grand Mosque.
State media reported others had begun arriving in the tent city of Mina on Mecca's outskirts where they will stay overnight before the hajj's high-point Thursday —prayers on Mount Arafat, where the Prophet Mohammed is believed to have delivered his final sermon.
About 1.4 million pilgrims arrived in Saudi Arabia ahead of the hajj, one of the five pillars of Islam that must be performed at least once by all Muslims with the means.
Authorities have ratcheted up heat protection measures such as extra shade to avoid a repeat of last year, when 1,301 people died as temperatures hit 51.8C (125.2F).
On Wednesday, pilgrims will perform the tawaf — walking seven times around the Kaaba, which Muslims pray towards each day.
Before entering Mecca, pilgrims must first enter a state of purity, called ihram, which requires special dress and behaviour.
Men don a seamless shroud-like white garment that emphasises unity among believers, regardless of their social status or nationality.
Pilgrims arriving on buses had begun already trickling into Mina on Tuesday afternoon, greeted by staff offering them coffee and dates.
"I am so happy, it's such an amazing feeling," said Reem al-Shogre, a 35-year-old Saudi performing the pilgrimage for the first time.
Artificial intelligence
Following last year's lethal heatwave, authorities have mobilised more than 40 government agencies and 250,000 officials to improve protection.
Shaded areas have been enlarged by 50,000 square metres (12 acres), thousands of additional medics will be on standby and more than 400 cooling units will be deployed, Hajj Minister Tawfiq al-Rabiah told AFP last week.
Artificial intelligence technology will help process the deluge of data, including video from a new fleet of drones, to better manage the massive crowds.
Authorities said most of the deaths last year were among unregistered pilgrims who lacked access to air-conditioned tents and buses.
This year, they have cracked down on the unregistered, using frequent raids, drone surveillance and a barrage of text alerts.
Hajj permits are allocated to countries on a quota basis and distributed to individuals by lottery.
But even for those who can obtain them, the steep costs prompt many to attempt the hajj without a permit, even though they risk arrest and deportation if caught.
Large crowds at the hajj have proved hazardous in the past, most notably in 2015 when a stampede during the "stoning the devil" ritual in Mina killed up to 2,300 people in the deadliest hajj disaster.
