Nearly 3 million Muslims performing the annual hajj pilgrimage in Saudi Arabia climbed the rocky desert Mount Arafat on Monday, chanting that they have come to answer God's call.

The white-robed pilgrims began their ascent at dawn, covering the Mountain of Mercy at Arafat in an endless sea of white as their chants of "Labyek Allah" — or "Here I am, God, answering your calling" — reverberated.

The climb is one of the cornerstones of the pilgrimage, which is required from every able-bodied Muslim at least once in his life. It is the site where Islam's Prophet Muhammad delivered his farewell sermon, and Muslims believe on this day the doors of heaven open to answer prayers and grant forgiveness.

Saudi Arabia's top cleric, Grand Mufti Sheik Abdul-Aziz Al Sheik, called on worshippers in a midday sermon to espouse moderation and said Islam would survive what he described as attempts to defame the faith.

"Islam will remain strong in the face of the vicious campaign that is attacking it," he said at the Namira mosque. "Muslims, the dearest thing you have is your religion and your belief. ... Be proud of your religion; don't ever be ashamed of identifying our religion."

The hajj draws millions of worshippers each year, the sheer numbers a challenge in preventing stampedes at holy sites, fires in pilgrim encampments and the spread of disease.

This year Saudi authorities have taken new measures to improve crowd management, including opening a new light-rail system to transport pilgrims between the sites.