More than two million Muslims gather this week for the annual haj pilgrimage to Islam's holy city of Mecca, where Saudi authorities hope to minimize spread of the H1N1 virus and prevent any political demonstration.

The haj, one of the world's biggest displays of mass religious devotion and a duty for Muslims who can perform it, has been marred in the past by fires, hotel collapses, police clashes with protesters and deadly stampedes.

This year, the mainly Sunni Muslim kingdom is battling Shi'ite Yemeni rebels after they raided its territory, an issue that raises fears of possible protests by fellow Shi'ite Muslims during the rituals. Saudi Arabia bans public protests.

Riyadh warned earlier this month against any attempt to politicize the pilgrimage. The warning followed remarks by Iranian Supreme Leader Ayatollah Ali Khamenei to departing Shi'ite pilgrims that they could not ignore conflict in Iraq, Afghanistan, the Palestinian territories and Pakistan.

The haj should "display the firm resolve of the Muslim nation to confront attempts that damage its unity and progress," Khamenei said.

Interior Minister Prince Nayef, whose country has been battling al Qaeda militants since 2003, said late Sunday about 100,000 men are deployed to ensure security at holy sites. "We hope we will not have to resort to force."

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Riyadh is also trying to prevent a spread of the H1N1 virus as the crowded rituals provide an environment for transmission of the disease. At least four pilgrims have died of the virus since the beginning of the haj season.

Since the World Health Organization declared H1N1 a pandemic in June, experts fear pilgrims from some 160 countries would carry the virus initiating waves of outbreaks worldwide.

Riyadh has installed devices to detect infections at points of entry and set up a 300-bed clinic in the Red Sea port of Jeddah where most foreign pilgrims arrive.

On the climax of the haj Thursday, worshippers spend the day gathered en masse at Arafat near Mecca. The following day pilgrims begin casting stones at pillars over three days in a symbolic renunciation of the devil's temptation.

Authorities have improved facilities to ease the flow of pilgrims, particularly around the area where crowds gather to throw stones at the pillars. In 2006, 362 people were crushed to death there, the worst haj tragedy in 16 years.

Saudi Arabia has built a four-storey platform around the pillars to expand the access area.