The Hajj, the five-day annual pilgrimage undertaken by Muslims to the holy city of Mecca, is currently underway.

The journey to Saudi Arabia results in the world's largest single gathering of people and this year began on the evening of Sunday 19 August and ends on Friday 24 August.

Here's everything you need to know.

What is it?

One of the Five Pillars of Islam, the Hajj is a sacred duty every Muslim must undertake at least once in their lifetime (unless they are prevented from doing so by ill health or financial hardship).

The Quran traces its origins to the story of Abraham, ordered by Allah to leave his wife Hajara and infant son Ishmael in the desert of ancient Mecca as an act of faith. Hajara searched frantically for water, passing between the hills of Safa and Marwah seven times, but found none, only for a fresh spring to burst from the ground beneath the child's foot.

Abraham constructed the Kaaba on the spot, the black stone cuboid at the the centre of what became the Great Mosque, around which worshippers walk counter-clockwise seven times in solemn contemplation of their faith as part of the Tawaf.

The Prophet Muhammad later marched from Medina to Mecca, his own birthplace, to rid the Kaaba of pagan idols in 632, reconsecrating it in honour of Allah.

The pilgrimage in homage to the miracle and its legend today serves to unite Muslims and remind all followers of Islam that they are equal in the eyes of god.

As such, pilgrims strip themselves of the material trappings of wealth and status and wear the plain white seamless robes of Ihram to indicate their spiritual state.

In addition to circling the Kaaba, pilgrims say two Rakaat prayers at the Place of Abraham, re-enact Hajarah's hunt for water between Safa and Marwah, drink from the Zamzam Well, stand vigil at Mount Arafat and on the plains of Muzdalifa and cast stones at three pillars, a symbolic act representing evil being driven from the hearts of men.

When is it?

The Hajj takes place between the eighth and 13th day of Dhu al-Hijjah, the date of which alters every year by the Gregorian calendar because the Islamic lunar calendar is 11 days shorter.

Eid al-Adha also falls on the 10th day of the month. This is a Festival of Sacrifice in honour of Abraham's willingness to sacrifice Ishmael at Allah's request, a supreme act of faith, which he was prevented from having to carry out by the deity providing him with a goat to slaughter in his son's stead.

Today, the same animal is sacrificed in memory of the story. It is cut into thirds: one portion is given to the impoverished, another to friends and relatives and the last retained and eaten by the family.

Is the Hajj safe?

Two million pilgrims are expected to take part this year, thousands of whom are embarking from the UK.

While the occasion is a holy one, overcrowding led to a fatal stampede in 2015 which killed over 2,000 pilgrims and Hajj fraudsters attempting to con visitors remain a real problem.

“Some will arrive in Saudi Arabia to discover the accommodation they booked does not exist while others will find that their whole trip is in fact a scam set up by illegitimate travel operators that have disappeared with thousands of pounds of their money,” the Metropolitan Police have warned.

Advice on staying safe includes making sure you travel with a reputable tour operator who is ATOL-protected and part of a recognised trade association such as ABTA. All members of the latter have to follow a code of conduct, reducing the risk of fraud.

Ensure you have all of your terms and conditions in writing and make sure your flight details, accommodation and Hajj visa are all valid, experts advise.

In pictures: Muslim pilgrims gather on Mount Arafat for Hajj Show all 19 1 /19 In pictures: Muslim pilgrims gather on Mount Arafat for Hajj In pictures: Muslim pilgrims gather on Mount Arafat for Hajj Muslim pilgrims gather on Mount Arafat (or the Mount of Mercy), where the Prophet Muhammad is believed to have delivered his final sermon Reuters In pictures: Muslim pilgrims gather on Mount Arafat for Hajj A Muslim pilgrim prays as another takes a photo on the Mount Arafat AP In pictures: Muslim pilgrims gather on Mount Arafat for Hajj Muslim pilgrims arrive at the plains of Arafat Reuters In pictures: Muslim pilgrims gather on Mount Arafat for Hajj Muslim pilgrims arrive at the plains of Arafat AP In pictures: Muslim pilgrims gather on Mount Arafat for Hajj Mount Arafat during the Hajj EPA In pictures: Muslim pilgrims gather on Mount Arafat for Hajj Muslim pilgrims pray on Mount Arafat (or Jabal ar-Rahmah) during the Hajj AP In pictures: Muslim pilgrims gather on Mount Arafat for Hajj Muslim pilgrims pray by the Namira Mosque on the plains of Arafat during Hajj Reuters In pictures: Muslim pilgrims gather on Mount Arafat for Hajj Muslim pilgrims pray by the Namira Mosque on the plains of Arafat during Hajj Reuters In pictures: Muslim pilgrims gather on Mount Arafat for Hajj Muslim pilgrims gather on Mount Arafat (or the Mount of Mercy) during the Hajj AFP/Getty In pictures: Muslim pilgrims gather on Mount Arafat for Hajj A Muslim pilgrim wheels his children on a wheelchair towards Mount Arafat AP In pictures: Muslim pilgrims gather on Mount Arafat for Hajj Pilgrims circle the Kaaba as they pray in the Grand Mosque of Mecca ahead of the Hajj AP In pictures: Muslim pilgrims gather on Mount Arafat for Hajj Muslim pilgrims arrive at the plains of Arafat Reuters In pictures: Muslim pilgrims gather on Mount Arafat for Hajj Muslim pilgrims gather on Mount Arafat (or Jabal ar-Rahmah) during the Hajj EPA In pictures: Muslim pilgrims gather on Mount Arafat for Hajj Muslim pilgrims gather on Mount Arafat (or the Mount of Mercy) during the Hajj AFP/Getty In pictures: Muslim pilgrims gather on Mount Arafat for Hajj Muslim pilgrims shelter from the sun under a bridge near Mecca EPA In pictures: Muslim pilgrims gather on Mount Arafat for Hajj Muslim pilgrims walk towards Namirah mosque on Mount Arafat during the Hajj AP In pictures: Muslim pilgrims gather on Mount Arafat for Hajj Muslim pilgrims gather on Mount Arafat (or Jabal ar-Rahmah) during the Hajj AFP/Getty In pictures: Muslim pilgrims gather on Mount Arafat for Hajj Muslim pilgrims gather on Mount Arafat (or the Mount of Mercy) during the Hajj AFP/Getty In pictures: Muslim pilgrims gather on Mount Arafat for Hajj Muslim pilgrims gather on Mount Arafat (or Jabal ar-Rahmah) during the Hajj AFP/Getty

You should make sure your travel insurance covers all aspects of the journey and that the appropriate vaccinations have been taken. Researching the local laws and customs is also recommended.

Consular staff can be contacted on a 24-hour helpline in the case of emergencies during the Hajj on: +966 5010 04268.