More than 1.5 million Muslims are gathering for the annual hajj pilgrimage to Mecca, where Saudi Arabian authorities have boosted security amid concerns about demonstrations against a possible U.S.-led war in Iraq.

Saudi officials expect 500,000 pilgrims from inside the kingdom to join more than 1.3 million foreigners in the five-day ritual starting Saturday.

The Bush administration heightened its terror alert status Friday, saying intelligence pointed to a possible attack timed to coincide with the hajj. And authorities in the Persian Gulf expressed concern about possible violent protests since many Muslims consider the threat of war against Iraq and American support for Israel to be part of a campaign against their faith.

"The Iraqi crisis will have its repercussions among the pilgrims who will make their views heard to the world," Abbas Ali Hosseini, a Shiite Muslim cleric in Qom, Iran, told The Associated Press.

At a news conference outlining preparations for this year's hajj, Saudi Interior Minister Prince Nayef said his country "won't allow anybody to mess with security.

"We have increased our security measures and taken all the (necessary) precautions," he said, according to the official Saudi Press Agency.

A pilgrimage to Mecca, birthplace of the Prophet Muhammad, is required of all able-bodied Muslims who can afford the trip. There are more than 1 billion Muslims worldwide.

Jonathan Stevenson, an anti-terrorism expert at the International Institute for Strategic Studies in London, said there were ideological and logistical reasons why the hajj could be a period of high risk for terror attacks.

"There's the chance that ... with more Muslims traveling, monitoring systems would suffer overload and therefore there would be more likelihood of somebody slipping through the cracks," he said.

The pilgrimage also could draw attention to American troops in Saudi Arabia, one of the main complaints of Usama bin Laden and his supporters, Stevenson said.

U.S. Attorney General John Ashcroft cited an "increased likelihood" that Al Qaeda would attack Americans, either at home or abroad, as the reason for raising the national terror alert from yellow to orange, the second-highest level.

Last year, during the first hajj since the Sept. 11 attacks, the Saudi government deployed tens of thousands of police and installed digital eye-scanning and fingerprinting machines to collect data on pilgrims.

Every year, authorities and clerics call on pilgrims to put politics aside during the hajj.

"Hajj provides a sense of belonging when the pilgrims gather ... and pray at the same time. This sense of belonging to Islam should help them think and work for the good and sake of Islam in all domains after they go back home," said Ali el-Simman, vice president of a leading interfaith dialogue committee in Cairo, Egypt.

The hajj originated in pre-Islamic days, when Arabs gathered in Mecca to worship idols, trade and reconcile tribal differences. Those gatherings attracted poets whose work survives today as some of the best Arabic verse.

The hajj rituals begin with the pilgrims, dressed in white, visiting the Grand Mosque to circle the Kaaba, the large cubic stone structure that Muslims face during their five daily prayers.

The next day, the climax of the hajj, they gather at nearby Mount Arafat — where Muslims believe Muhammad gave his last sermon — and chant in unison, "Here I am, oh Almighty, here I am."

The following day involves "stoning the devil" in Mina, when pilgrims throw pebbles from giant ramps surrounding three pillars symbolizing the devil. They then slaughter a camel, sheep or cow to mark the beginning of Eid al-Adha, or "Feast of the Sacrifice."

The feast commemorates God's provision of a ram to substitute for Abraham's impending sacrifice of his son, and is considered the most important holiday in the Islamic calendar.

The pilgrims remain in Mina for two more days to perform the stoning ritual again, then circle the Kaaba again before leaving.

Deaths often accompany the hajj rituals. In 1987, more than 400 people were killed in Mecca when security forces clashed with Iranians staging an anti-U.S. demonstration.

Last year, about 35 people died in a stampede while performing the "stoning of the devil" ritual. A 1997 fire in Mina killed more than 340 pilgrims.

The most deadly hajj-related tragedy was a 1990 stampede that killed 1,426 pilgrims.