Pilgrims in white robes and sandals packed the ancient city after flooding in on planes, buses and trains for the annual rites.
This year's hajj - one of the world's biggest annual religious gatherings, with a tragic history of stampedes and other disasters - could break attendance records, officials said.
"As the hajj draws near, the Kingdom of Saudi Arabia prepares... for the largest Islamic gathering in history," Minister of Hajj and Umrah Tawfiq Al-Rabiah said in a video published by the ministry this week.
More than two million people from more than 160 countries will attend, Rabiah said - a dramatic increase on the 926,000 from last year, when numbers were capped at one million following the COVID-19 pandemic.
About 1.5 million pilgrims from abroad had already arrived by Wednesday evening, Saudi authorities have said.
Travelers from around the world have been pouring into Jeddah's modernized airport, some of them using streamlined visa services to disembark from planes straight onto buses to their accommodation.
Some 24,000 buses will be in service to ferry the pilgrims, as well as 17 trains capable of moving 72,000 people every hour, officials said.
This hajj will be the biggest since the requirement for women to be accompanied by male guardians was dropped in 2021.
This year, the maximum age limit has also been scrapped, meaning thousands of elderly will be among those contending with Saudi summer temperatures that are expected to reach 44 degrees Celsius (111 Fahrenheit).
Rites include circling the Kaaba, the large black cube in Mecca's Grand Mosque, praying on Mount Arafat and "stoning the devil" by throwing pebbles at three giant concrete walls representing Satan.
In 2019, about 2.5 million people took part. Only 10,000 were allowed in 2020, at the height of the coronavirus pandemic, rising to nearly 59,000 a year later.
The hajj reflects the heart of Islam, and is one of the "five pillars" of the faith.
"It is an unbelievable feeling that is very emotional," Souad bin Oueis, a 60-year-old Moroccan pilgrim, told AFP after arriving on her first visit to Saudi Arabia along with her husband.
The hajj rituals begin late Sunday at the Grand Mosque in Mecca. The worshippers will sleep in tents on Monday night and spend Tuesday at Mount Arafat, the climax of the hajj, where the Prophet Mohammed is believed to have delivered his final sermon.
After casting pebbles in the "stoning of the devil" ritual on Wednesday, marking the start of the Eid al-Adha holiday, pilgrims return to Mecca to perform a farewell "tawaf" - circling seven times around the Kaaba.
Mecca pilgrimages are a major source of income for Saudi Arabia, which is embarking on an ambitious plan to overhaul its largely oil-dependent economy. The hajj and year-round umrah rituals generate an estimated $12 billion annually.
An expansion project that involves scaling up infrastructure and transport supporting Mecca and Medina, where two of Islam's holiest sites are located, is a key part of the economic plan as the kingdom looks to increase visitor numbers.
This year's summer timing for the hajj, which follows the lunar calendar, will test the endurance of worshippers during the four-day, mostly outdoor ritual.
More than 32,000 health workers will be on hand to help fend off heatstroke, dehydration and exhaustion.
But severe weather is just one of the risks at an event that has been hit by deadly incidents from stampedes to militant attacks.
Just eight years ago in 2015, as many as 2,300 worshippers died in a stampede during the "stoning of the devil" ritual in Mina near Mecca, the worst hajj disaster ever.
More than 700 people died in a notorious attack in 1979, when hundreds of gunmen demanding the abdication of the Saudi royal family barricaded themselves inside the Grand Mosque, taking dozens of pilgrims hostage.
Umer Karim, a doctoral researcher focusing on Saudi foreign policy at Britain's University of Birmingham, said hosting the hajj is a matter of prestige and a source of political legitimacy for Saudi rulers, the custodians of the holy sites.
The pilgrimage "gives the kingdom and its rulers a huge amount of esteem and prestige which then becomes a major source of the kingdom's soft power," Karim stated.