The Saudi government faces two challenges, namely the danger of the A-H1N1 flu virus and possible tensions between Sunnis and Shias, which in the past caused hundreds of deaths and injuries.

Local health authorities are on high alert; so far, 20 pilgrims have shown signs of the flu. On Saturday, four of them died; they had prior health problems like pulmonary infection and cancer. Another four remain in care, but and 12 were dismissed from hospital.

Over the past few weeks, Riyadh has urged other governments to restrict the pilgrimage to healthy adults between 18 and 65.

At the same, Saudi authorities deployed additional doctors. They have also provided more hospital beds and stockpiled 1.5 million units of the anti-viral drug Tamiflu, the most widely used drug in the fight against the flu.

A Saudi doctor said that the authorities were hoping that at least 20 per cent of pilgrims would have been vaccinated before arriving. Still, six 24-hour health clinics are available within the Grand Mosque.

However, the flu is not the authorities’ only concern; possible violence is a swell. In recent weeks, the Saudi government called on Iran not to politicise Hajj after Iran’s Supreme Leader Ali Khamenei called for “demonstrations against the United States and Israel.”

Saudi Interior Minister Nayef Ben Abdel said he hoped there would be no need to use force to ensure pilgrims’ safety.

General Mansur al-Turki, the interior ministry official in charge of Hajj security, said he did not expect trouble, but warned that protests "are prohibited in the Hajj and we will not let them take place".

In the past, the greater pilgrimage to Islam’s holiest sites was marred by serious incidents resulting in hundreds of dead. In 1987, a police crackdown against an anti-US and anti-Israel demonstration by Iranian pilgrims left 402 people dead. Tehran responded by boycotting Hajj for three years.