A pilgrim attending the hajj speaks on a mobile phone amidst heavy rains Wednesday in Mecca, Saudi Arabia. The heaviest rainstorms to hit Islam’s annual hajj in years soaked pilgrims and flooded the road into Mecca, snarling traffic as millions of Muslims headed for the holy sites and added an extra hazard on top of intense concerns about the spread of swine flu.

Jiddah, Saudi Arabia ? Muslim pilgrims circled Islam’s holiest site Wednesday in their traditional white robes, with a few additions — umbrellas and face masks — as the opening of the annual hajj was complicated by torrential rains and fears of swine flu.

Saudi authorities have been planning ways for months to inhibit the spread of swine flu during the pilgrimage, which is seen as an incubator for the virus. The four-day event is one of the most crowded in the world, with more than 3 million people from every corner of the globe packed shoulder to shoulder in prayers and rites.

Now they are scrambling to deal with sudden, unexpected downpours that could worsen one of the gathering’s perennial dangers: deadly stampedes.

In 2006, all it took was a dropped piece of luggage to trip up a crowd and cause a pileup that killed more than 360 people at one of the holy sites. The rains also could cause flash floods or mudslides in the desert mountains where most of the rites take place.

On Wednesday, the only fallout from the rains were epic traffic jams, flooded tents and washed-out streets as the faithful tried to make their way to the Kaaba in Mecca. To kick off the hajj, Muslims circle seven times around the cube-shaped shrine draped in black cloth.

At times, crowds of men and women under umbrellas, some wearing surgical masks against the flu, circled in the courtyard of the mosque surrounding the shrine. But at other times during the day, the site was nearly empty, as were the surrounding streets, which would typically be jammed on the hajj’s opening day.

Many travelers were struggling just to get to the site. Floods closed down part of the main road to Mecca from the Red Sea coastal city of Jiddah — the entry point into the country for most of the faithful. As a result, cars were backed up as far as 20 miles, nearly half the highway’s length.

Streets were flooded in Jiddah, waist high in some areas, and some pilgrims and journalists were trapped there. In Mecca, electricity was off and on throughout the day.

It often rains in Mecca and Jiddah during the winter months, but Wednesday’s downpour was the heaviest in years during the hajj. Jiddah was swamped with 2.76 inches of rain, more than it would normally get in an entire year, according to Dale Mohler, senior meteorologist at the Web site, Accuweather.com.

Rains were expected through Friday, he said, warning of the possibility of flash floods and mudslides in the mountains surrounding Mecca.

“There’s no vegetation on the slopes to soak up the rain,” Mohler said.

The hajj is a religious duty for every able-bodied Muslim. Many around the world wait a lifetime for a chance to make the spiritual journey in the footsteps of the Prophet Muhammad and Abraham, who Muslims view as a forefather of Islam. For believers, it is an opportunity to cleanse one’s sins before God.

It is also a logistical nightmare. Over four days, the population of a small city moves by car, bus and foot between Mecca and several holy sites in the desert nearby, each day performing a different rite all at the same time.

Civil Defense spokesman Maj. Abdullah al-Harthi said his organization has plans ready to deal with flooding, and had 300 buses to evacuate people if necessary.

Saudi Arabia’s biggest worry for months ahead of the hajj was swine flu. The Saudi government has been working with the United States’ Centers for Disease Control and Prevention to set up clinics and take precautions to stem any outbreak.

Signs at the airport and around the holy sites urge the faithful to cover their faces when they cough, wash their hands often and wear a mask. The swine flu vaccine is available free at the airport. More than 100 clinics have been set up at holy sites, and large supplies of Tamiflu and other anti-flu medications are on hand.