The millions of people must perform a litany of rituals in five days, including the symbolic stoning of the devil in Mina, about 2 miles away from the Mecca holy site.
That's where the deadly stampede took place Thursday, the third day of the five-day event. Officials say 717 people died and 900 were injured.
Iran's semiofficial FARS news agency said that 131 Iranians were among the dead. Another 365 remained missing, the state-run IRNA news agency reported, cautioning against assuming they have died.
The dead were from at least 13 nations.
India's foreign minister reported on Twitter that 14 Indian citizens had died. Egypt counted 14 deaths and at least 30 people injured. Somalia reported seven of its citizens were dead; Pakistan, six; Senegal, five; Turkey, four; and Kenya and Algeria, three each. Two people from Indonesia and two from Nigeria were reported killed, along with one each from the Philippines and the Netherlands.
Saudi Arabia said six members of its civil defense forces were injured in the stampede.
The Muslim Council of Wales said that it had heard from only two of six tour groups that had traveled from south Wales for the Hajj. Together, the groups number about 250 pilgrims, council official Saleem Kidwai said. Families and relatives are "very anxious, very concerned," he said.
Hajj pilgrim Ethar El-Katatney, a journalist and blogger, said people were trying to push their way in opposite directions -- some headed to the site of the stoning, some coming back from their previous ritual.
"As our group started to head back, taking Road 204, another group, coming from Road 206, crossed our way," said another worshipper, Ahmed Mohammed Amer.
"Heavy pushing ensued. I'm at a loss of words to describe what happened. This massive pushing is what caused the high number of casualties among the pilgrims."
After the stampede, it took hours for rescue workers to try to tend to all those trampled.
"The ambulances, the sirens were overwhelming," El-Katatney said. "For hours and hours, you could hear them constantly."
El-Katatney said the sight of the carnage was simply "horrendous."
"It's literally a pile of bodies of people who ... pushed, they shoved, they panicked, they screamed," she said. "It was hot, someone fell, others trampled and they got stampeded."
Time pressures may have contributed to the disaster, El-Katatney said.
"There's so little time to complete the rituals," she said.
Journalist Khaled Al-Maeena said he believes pilgrims rushing to finish could have been the main reason for the stampede.
"People like to do the first stoning in the morning," he told CNN from Jeddah, Saudi Arabia.
Extreme heat and exhaustion
The journey is physically grueling enough on its own.
But with temperatures soaring over 43 degrees Celsius (110 degrees Fahrenheit), anyone who succumbs to the elements might collapse and never recover, El-Katatney said.
"I was out for a couple of hours just kind of taking photos, recording. And just two hours standing in the sun makes you so dizzy and so incredibly faint," she said from Mina.
"But regardless, people were still continuing to ... their ritual, where the stampede happened."
El-Katatney said she talked to some of the men who were caught in the mayhem.
"They told me how if you fell, if you weren't strong enough to withstand the pushing and shoving ... if you fell, you weren't going to get up again."
Inexperience and confusion
Even though Saudi officials are extremely versed in hosting Hajj crowds, many of the pilgrims are making the journey for the first time and might not be prepared to follow all directions or handle the chaos.
"If any mistake happens -- if a group makes the wrong turn -- that will cause a disaster," Jamal Khashoggi of Saudi Arabia's El Arab TV told CNN. "And that's exactly what happened yesterday."
Maj. Gen. Mansour Al-Turki, the Saudi Interior Ministry's security spokesman, hinted that the problem may have stemmed from some pilgrims not following established guidelines, the Saudi Press Agency reported.
And novice pilgrims might try to "go on their own, or try to take a shortcut," Khashoggi said.
Hezbollah leader Hassan Nasrallah told Lebanese al-Manar TV that Saudi Arabia should let the management of the pilgrimage be a joint Muslim issue or allow a Muslim committee to supervise its management.
A deadly history
Hundreds of other pilgrims have been killed
during the same ceremony in years past. But Thursday's disaster was the deadliest at Mina since 1990, when 1,426 people died.
The latest calamity also came 13 days after a crane collapse killed more than 100 people
at another major Islamic holy site, the Grand Mosque in Mecca.
After a stampede during Hajj killed 363 people in 2006, the Saudi government erected three massive pillars and completed a $1.2 billion, five-story bridge
near the site where pilgrims can toss stones.
But after the latest mass tragedy, many are wondering what more can be done to prevent another disaster.
In remarks Thursday, Pope Francis
offered his condolences.
"My sentiments of closeness in the face of the tragedy that they suffered in Mecca. In this moment I give them assurances of my prayers. I unite myself with you all in prayer to the Almighty God, all merciful."