The world's largest annual religious pilgrimage began on Wednesday overshadowed by the swine flu pandemic as 2.5 million Muslims amassed at the holy city of Mecca for the hajj. A rare rainstorm and possible banned protests by Iranian pilgrims also threatened to mar the four-day hajj, but Saudi officials said they are prepared for all eventualities. A sea of pilgrims from all over the world, dressed in flowing white robes, surrounded the Kaaba shrine inside Mecca's Grand Mosque for dawn prayers.