Masses of pilgrims in Saudi Arabia circle the Kaaba ahead of the official beginning of the Hajj

Masses of Muslim pilgrims in the Saudi city of Mecca on Thursday circled the Kaaba, Islam’s holiest site, a day before heading to the nearby desert area of Mina to officially open the Hajj, the annual Islamic pilgrimage that takes the faithful along a route taken by the Prophet Muhammad.

The area around the Grand Mosque in Mecca was overcrowded by midday, with the faithful going back and forth to the holy site to pray and make the circumambulation, or Tawaf in Arabic.

More than 1.5 million pilgrims had arrived in the country as of Tuesday, according to Saudi authorities. More are expected, and hundreds of thousands of Saudis and others living in Saudi Arabia will also join them when the pilgrimage officially begins on Friday.

Since their arrival in Mecca, the pilgrims have performed the circumambulation. The large crowds circling the Kaaba will last into Friday, the Hajj’s first day, when pilgrims move to Mina, a desert plain outside the city.

They will then head to the Mountain of Arafat for a daylong worship. Afterwards, they will move to Muzdalifah, a rocky plain area a few miles away. There, pilgrims collect pebbles to be used back in Mina, in the symbolic stoning of pillars representing the devil.

One of the world’s largest religious gatherings, the Hajj is one of the five pillars of Islam. All Muslims are required to undertake it at least once in their lives if they are physically and financially able to do so.

Those in the Hajj view the pilgrimage as an opportunity to strengthen their faith, wipe out old sins and start anew.