It is one of the biggest annual gatherings of people on the planet.
And in 2012, when three million people were present – the highest number ever registered – the Hajj became the tenth largest gathering in human history.
More than two million believers come to Saudi Arabia for this year's pilgrimage to Mecca – known in Arabic as Makkah.
The hajj takes place this year from Sunday, August 19 to Friday, August 24, with most participants traveling on two- or three-week travel packages.
Before the pilgrimage began, the faithful visited Hira Cave on Jabal Al-Noor Mountain. Here the Prophet Muhammad received the first revelations of the Qur'an.
The Hajj is a journey that all Muslims should make at least once in their lives if they are physically and financially capable of doing so.
The British Hujjaj (Pilgrims) UK Association, located in the Sparkbrook area of Birmingham, has told Birmingham Live to wish everyone a "safe and enjoyable journey".
Khalid Pervez, general secretary of the association, which campaigns for the well-being and well-being of pilgrims, said: "Our sincere prayers and wishes are addressed to all pilgrims from all over the world, for a safe journey, successful and accepted hajj and a safe to theirs Love to return. "
Hajj is one of the five pillars of Islam besides faith, prayer, charity and fasting.
Mr. Pervez explained that millions of Muslims from all over the world gather in Mecca to "perform the same rituals while being similarly dressed while standing together at eye level".
"This makes the Hajj pilgrimage a true reflection of the dignified principles of equality, modesty and human brotherhood that are at the center of Islam," he said.
"The diversity of Muslim pilgrims in the Hajj is a true reflection of the cross-border and intercultural nature of Islam."
He added, "Hajj gives Muslims from all nations, races and countries the opportunity to get to know each other better, to learn more about each other's problems and concerns, and emphasizes the Islamic teachings of compassion, cooperation, tolerance and care for others . "
The association also thanked the Kingdom of Saudi Arabia for its efforts to promote the well-being and well-being of pilgrims.
But it says that pilgrims lack awareness of health and safety, and has called for compulsory training to prevent the tragedies of recent years.
It also warned people to pay attention to the deadly health risks of heat stroke and heat exhaustion. Temperatures in Saudi Arabia in August can reach 45C.
How did the Hajj start?
The event is linked to the Islamic Prophet Muhammad, whose journey from Medina to Mecca became the first Hajj in the 7th century.
But it can actually be traced back thousands of years earlier at the time of Ibrahim, who built the Kaaba – the cube-shaped building in the heart of the Great Mosque of Mecca – and invited people to make a pilgrimage there.
Millions of Hajj pilgrims gather to circle the Kaaba seven times counterclockwise.
What happens every day of Hajj?
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Day 1 – Pilgrims walk around the Kaaba in the ritual called Tawaf, perform prayers, walk or walk seven times between the hills of Safa and Marwah, and then make their way to the tent city of Mina, where they stay all day and several Sets play prayers.
Day 2 – Pilgrims go from Mina to the plain of Arafat, where they are standing in a vigil, forgetting sins, seeking mercy and listening to sermons. After sunset, they set off for Muzdalifah to spend the night in the open air collecting pebbles for the Devil's Stoning Ritual.
Day 3 – The faithful return to Mina for the ritual of stoning the devil. They throw seven stones at each of the three walls to symbolize what Ibrahim did when the devil tried to stop him from sacrificing his son. Ibrahim was instead given a lamb to slaughter, and this is the origin of Eid ul Adha, the feast of the sacrifice.
Day 4 and 5 – Muslims repeat the ritual Stony of the Devil and then return to Mecca. If the pilgrims can not leave Mina before sunset on the 5th day, they must stay one more day and perform the stoning ritual again.