The hajj in numbers

The hajj pilgrimage, which started at Islam's holiest sites in Saudi Arabia on Friday, is one of the world's largest annual gatherings.

Here are key figures: - Some 2.5 million pilgrims are expected to take part this year, according to Saudi media, compared to 2.37 million last year, 1.86 million in 2016 and just 24,000 in 1941.

- Tens of thousands of security personnel, including more than 17,000 civil defence employees, are deployed to help safeguard pilgrims, officials say. Thousands of security cameras have been set up along the pilgrimage route.

- Twenty-five hospitals, 156 health centres and more than 30,000 health practitioners have been prepped to provide emergency services to pilgrims, according to state media and the hajj ministry.

- Some 350,000 air-conditioned tents have been set up to house pilgrims, a Saudi official said. More than 5,600 hotels and apartment buildings in Mecca and Medina -- with a capacity to host 2.7 million people -- are dedicated to pilgrims, according to state media.

- Around 3,000 international flights have transported pilgrims to the kingdom, according to official figures. Some 25,000 buses have also brought the faithful to Mecca. - More than 13,000 telecommunication towers and more than 5,400 WiFi zones have been set up to serve pilgrims, the hajj ministry says.

- Some 4,000 cleaners, equipped with 400 machines, are deployed to clean the Grand Mosque in Mecca, according to authorities.

- More than one million livestock are readied for sacrifice by hajj authorities, according to state media.

- The kingdom hopes to welcome 30 million pilgrims annually in the kingdom by 2030. Muslims also travel to Saudi Arabia for the umra pilgrimage, which can be performed at any time of the year.

- Around 96 million pilgrims have attended the hajj over the past 50 years, according to official figures.