More than two million Muslims began the annual haj pilgrimage.
Amid heightened security in Saudi Arabia, Sheikh Saleh al-Taleb urged Muslims gathered for Friday prayers against the use of violence.
"The haj should not be turned into anything against its original intentions. A Muslim should not hurt another Muslim," he said in Mecca's Grand Mosque, the focus of the pilgrimage.
Diplomats say the Saudi government is worried the militants will strike during the haj to undermine the ruling family, whose authority stems from its custodianship of Islam's holiest sites.
Yesterday, Saudi security forces captured a wanted militant and a number of other suspects after a firefight in Riyadh in which five policemen were killed.
Securing the haj is the top priority of the 5,000 Saudi police and military deployed in and around Mecca.
The security forces are also in charge of controlling the vast sea of pilgrims, after stampedes and crushes killed hundreds of people over the last decade, including 14 last year.
Saudi Arabia insists the five-day haj, one of the most striking manifestations of faith and unity in the world today, should be solely a religious affair.
But many pilgrims have politics on their mind. "We hope God will give success to the Muslim people around the world and especially in our region," said Iraqi pilgrim Qadir Khidr. "And we hope God will get us out of our crisis now like he got us out of the last one."
Travelling by foot, public transport and private cars, the pilgrims will stream through a mountain pass to Mena, some three kilometres outside Mecca. In a massive logistical operation some 20,000 buses will transport most of the pilgrims to the plains outside Mecca.
On Saturday, they move to nearby Mount Arafat for a day of prayers in commemoration of the Prophet Mohammad's farewell sermon 14 centuries ago. On Sunday the pilgrims throw stones at pillars on the spot where the Devil is said to have appeared to biblical patriach Abraham - the scene of many deadly stampedes.
The Muslim holiday of Eid al-Adha begins the same day, with the sacrificial killing of sheep, goats and cows in commemoration of Abraham's willingness to sacrifice his son Ismail at God's command.
A duty for every able-bodied Muslim at least once in a lifetime, the haj revolves around ritual circling of the Kaaba, a cube-shaped structure in the Grand Mosque which in Islam is a magical site where God's presence is most felt on earth.