Muslims from around the world began arriving in Saudi Arabia this week, with more than two million expected to take part in this year's haj - one of the world's biggest displays of mass religious devotion. Thousand of people gathered outside Mecca's holy Al Haram Grand mosque for prayer for the annual pilgrimage, a duty for every able-bodied Muslim to take part of at least once in a lifetime. Pilgrims converge on the Grand Mosque and follow a route around the rocky mountains of the ancient city in line with a tradition established by the Prophet Mohammed. "I am the happiest person on earth. Thank God, thank God, thank God. I wish we could do this every year, inshallah (God willing)," said Hajja Nabaweya Shehada Ameen, a pilgrim from Egypt. Saudi Arabia usually hosts some 1.5 million pilgrims from abroad, and grants visas to 500,000 inside the kingdom, but the number often swells to 2.5 million as people sneak in illegally. Stampedes and hotel collapses have marred the pilgrimage in recent years. In January, 362 pilgrims died in overcrowding at the Jamarat Bridge on the last day. That followed the death of 76 people when a hotel collapsed in Mecca before the rites began. The 2005 death toll was the worst since 1990 when 1,426 people were killed in a stampede in a tunnel in Mecca. The authorities say rigorous checks have been put in place in Mecca's hotels to avoid a repeat of the last tragedy. But this has not stopped millions of worshippers from flocking to the holy city to the take part in the haj. "May God give each person this opportunity (to do the pilgrimage). This is a beautiful journey. The greatness of this place, the greatness of this situation, it is marvellous. Everyone should see and be part of this," said Mohammad Salah from Syria. The interior ministry is expected to deploy over 50,000 men in the Mecca area to combat potential militants, demonstrators and disorderly behaviour that could lead to overcrowding. Overcrowding is a perennial worry but this time, regional tension between Shi'ites and Sunnis has heightened security concerns while authorities remain on the look-out for al Qaeda-linked militant violence -- a fear in recent years. The haj takes place in the shadow of violence between Sunnis and Shi'ites that has taken Iraq to the brink of civil war this year. Sunni-Shi'ite tension is also high in Lebanon, where Shi'ites are leading efforts to bring down a Sunni-led cabinet. Iranian and other pilgrims have used the haj for political protests in the past. Shi'ite Iran is at loggerheads with the West over its nuclear programme and backing Shi'ite groups in Arab countries, raising the potential for trouble at the haj. Saudi Arabia said earlier this month it had detained 136 foreign and Saudi militants, some posing as pilgrims, who were planning a series of suicide bombings and assassinations around the desert country of 24 million people.