Story highlights The pilgrims this year reportedly have come from a total of 183 countries

President Obama and his wife extend greetings for a happy Eid al-Adha

Hajj is an annual pilgrimage that lasts about five days

Millions of Muslims from around the world were in the holy city of Mecca on Saturday for the first day of Hajj, the annual pilgrimage that every able-bodied Muslim is expected to perform at least once in his or her life.

Dressed in white and following in the footsteps of the prophet Mohammed, pilgrims climbed Mount Arafat, just outside the Saudi Arabian city.

Hajj, a five-day pilgrimage, consists of a series of detailed rituals in Mina, Muzdalifa, Arafat and Mecca.

It is one of the five pillars of Islam.

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This year's pilgrimage is unique as it is the first to happen since the Arab Spring began last year.

Some experts are watching for potential flare-ups in Saudi Arabia, a country governed by an unelected royal family and where freedoms are limited.

The pilgrims this year have come from 183 countries, the Saudi Press Agency reported. About 54% of the pilgrims are male.

U.S. President Barack Obama released a statement of Saturday to coincide with the start of Hajj. Eid al-Adha, or the Festival of Sacrifice, begins on Sunday.

"Michelle and I extend our greetings for a happy Eid al-Adha to Muslims worldwide and congratulate those performing Hajj. Thousands of Muslim Americans are among those who have joined one of the world's largest and most diverse gatherings in making the pilgrimage to Mecca and nearby sites," he said.

"The Eid and Hajj rituals are a reminder of the shared roots of the world's Abrahamic faiths and the powerful role that faith plays in motivating communities to serve and stand with those in need. On behalf of the American people, we extend our best wishes during this Hajj season."