A huge security operation is being mounted in Saudi Arabia as millions of Muslims arrive for the hajj pilgrimage.
Thousands of troops, including anti-terror units, staged a show of force in the holy city of Mecca, where the event gets under way on Friday.
Saudi Interior Minister Prince Nayef bin Abdul-Aziz said security forces would deal with any trouble "firmly".
The warning follows a spate of bombings in Saudi Arabia by Islamic militants, which have killed scores of people.
'Ready for anything'
About two million people, most from abroad, are expected to attend this year's hajj.
Prince Nayef said forces were "ready for anything that could happen".
"We always say there is no guarantee that nothing could happen but we trust the security forces to be able to do their job," he told reporters.
About 5,000 troops, including snipers, bomb squads and border guards, marched on the plain of Arafat - one of the pilgrimage sites - as helicopters flew overhead.
Security is a top priority for the Saudis, who have been fighting Islamic militants in the desert kingdom in recent months.
Authorities are also concerned about possible anti-American riots during the hajj, the first since the US-led war on Iraq, which was deeply unpopular with many Muslims.
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