MECCA, Saudi Arabia (AFP) – The world’s largest annual religious pilgrimage began on Wednesday but was overshadowed by the swine flu pandemic as 2.5 million Muslims massed 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 were 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 Wednesday.

Four pilgrims, all of them already suffering from other health problems, had died from swine ahead of the official start of hajj rites.

But proven and suspected infections from the A(H1N1) flu amid hajj participants only number 67, Saudi health ministry spokesman Dr. Khaled Marghlani told AFP.

“Everything is going smoothly, thanks to God,” he said.

The flu has killed some 6,750 people around the world this year, the WHO said on Friday, and Saudi authorities have deployed as many as 20,000 health workers to deal with major outbreaks during the hajj.

Marghlani said the threat of heavy rain in Mecca could raise the health risks for pilgrims, but that “We have planned for this possibility.”

Amid a war of words between Riyadh and Tehran over predominantly Sunni Muslim Saudi Arabia’s treatment of Iran’s mainly Shiite pilgrims, sources in the Iranian delegation said they would undertake a ritual protest in their own camp in the massive pilgrims’ tent city in Mina valley.

While Saudis have banned any political acts that may disrupt the hajj, the Iranians said they will go ahead with the ritual chanting of “death to Israel, death to the United States” and call for unity among Muslims around the world.