This is the first time since the Covid pandemic that Saudi authorities allowed it to return to full capacity.
The gathering follows the lunar calendar, and this year fall in the middle of the summer.
According to Saudi officials, as temperatures soared to 48C (118F) on Thursday, 1,700 cases of heat stress were recorded.
As the events are held outdoors – mainly at Mount Arafat – authorities urged pilgrims to stay out of the sun and drink plenty of water.
Many of those performing the Hajj are pensioners — having saved for years to make the pilgrimage — and thus are vulnerable to the heat.
Exposure can cause dizziness, dehydration, exhaustion, cramps, headaches and rashes.
It is reported that at least 200 people have died during this Hajj, but many have suffered from heart and respiratory ailments.