More than two million Muslims from around the world began the hajj pilgrimage at Islam's holiest sites Wednesday, a religious duty and an epic multi-stage journey. This year sees pilgrims from Shiite Iran return to Mecca in Saudi Arabia after a hiatus following a diplomatic spat between the regional rivals and a deadly stampede in 2015. It also comes with the Gulf mired in a major political crisis that has seen thousands of faithful who would usually make the journey from neighboring Qatar stay away.

Riyadh and Tehran cut ties months later, after the execution of a Shiite cleric in Saudi Arabia sparked attacks on Saudi diplomatic missions in Iran. Iranian pilgrims were absent from last year's hajj for the first time in decades after the regional rivals failed to agree on security and logistics.

Muslim pilgrims marked the start of the hajj pilgrimage by circling the cube-shaped Kaaba in Mecca — Islam's holiest site — and performing a series of rites that trace the footsteps of the Prophet Muhammad (Peace Be Upon Him). The Kaaba represents the metaphorical house of God and the oneness of God in Islam. Observant Muslims around the world face toward the Kaaba during the five daily prayers.

The hajj is required of all Muslims once in a lifetime. The physically demanding journey tests pilgrims' patience as they withstand long waits and thick crowds on their path to achieving spiritual purification and repentance.

The journey of the five-day-long pilgrimage begins for many when they depart from their countries dressed in "ihram." For men, that entails wearing only terrycloth, seamless white garments meant to represent unity among Muslims and equality before God. Women wear loose clothing, cover their hair and forgo makeup and nail polish to achieve a state of humility and spiritual purity.

After prayers in Mecca, pilgrims will head to an area called Mount Arafat on Thursday where the Prophet Muhammad delivered his final sermon. From there, pilgrims will head to an area called Muzdalifa, picking up pebbles along the way for a symbolic stoning of the devil and a casting away of sins that takes place in the Mina valley for three days.

Saudi authorities have mobilized vast resources including more than 100,000 security personnel to avoid a repeat of the stampede in 2015 in which nearly 2,300 people were killed. Iran alone reported 464 deaths, the highest toll among foreigners.