(Adds quotes, details) By Souhail Karam MECCA, Saudi Arabia, Dec 28 (Reuters) - Two million Muslims began streaming out of Mecca on Thursday at the start of a haj pilgrimage with Saudi authorities on alert to prevent deadly overcrowding as well as sectarian violence. Moving in cars, vans and on foot, a mass of white-robed pilgrims poured into a tented city in the valley of Mena to begin the gruelling five-day ritual, a duty for every able-bodied Muslim who can afford to make the trip. "This is a night of mercy for all Muslims," said Hassan Abu al-Fadl, an Egyptian engineer wearing the simple white robe worn by pilgrims as a sign equality and selflessness. "It's rare to see all these Muslims of different ethnic backgrounds together in one place with one purpose." In one of the world's biggest displays of mass religious devotion, pilgrims converge on the Grand Mosque in Mecca and follow a route around the mountains of the ancient city in line with a tradition established by the Prophet Mohammad. With such large crowds, Saudi Arabia normally deploys more than 50,000 security men to try to avoid deadly stampedes and attacks by Islamists fighting the U.S.-allied Saudi royals. The authorities say they will crack down hard on Muslims who try to sneak into Mecca without official permits -- a phenomenon that can swell numbers by 500,000 to more than 2.5 million. State media said a record 1.65 million pilgrims had come from abroad, a 6 percent rise on the last haj. Several hundred thousand inside Saudi Arabia usually receive permits too. STONING CONFUSION In January, 362 pilgrims were crushed to death due to overcrowding at the Jamarat Bridge during the last haj season. The toll was the worst on the bridge for 16 years and followed the death of 76 people in a hotel collapse in Mecca. Disaster was averted on Monday when helicopters evacuated pilgrims from a Mecca hotel where a fire broke out. Saudi Interior Minister Prince Nayef has said improvements introduced this year would prevent overcrowding at the bridge, which for three days from Saturday will witness the symbolic stoning of the devil, the most incident-prone haj ritual. Saturday, Sunday and Monday -- the last three days of the haj -- will be the key test of the new arrangements. Saudi Arabia has allocated $1.1 billion to expand the Jamarat Bridge into a multi-storey structure. The first phase, completed in time for this week's haj, allows up to 250,000 pilgrims to move across the bridge each hour. Clerics of Saudi Arabia's hardline Wahhabi brand of Islam have given conflicting signals on when to do the stoning. In a ruling aimed at reducing overcrowding, Grand Mufti Sheikh Abdulaziz Al al-Sheikh, the top state-appointed religious official, said it could be done any time during the three days. But in a fatwa earlier this week, respected independent Wahhabi cleric Abdullah al-Fozan said otherwise. "Stoning should not take place before midday because of a saying of the Prophet," he said in an opinion circulated on Web sites. The haj takes place in the shadow of violence between Sunnis and Shi'ites that has taken Iraq to the brink of civil war. 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 plans and its backing of Shi'ite groups in Arab countries, raising the potential for trouble at the haj.
