Some 2.5 million Muslim pilgrims are descending on Mecca for the annual hajj amid tight security and with new facilities, including a railway, to ease the occasionally fatal congestion. Saudi Interior Minister Prince Nayef bin Abdul Aziz has said he cannot rule out an Al-Qaeda attempt to disrupt the five-day ritual, which begins on Sunday, although the group has not attacked pilgrims in the past. Were it to do so, it would embarrass Saudi Arabia, which many Muslims consider the guardian of Islam's two holiest cities, Mecca and Medina, and which showcased its security forces in a military parade on Wednesday. The hajj is one of the five pillars, or requirements, of Islam, which must be performed at least once in their life by all believers who have the health and the means to do so. Final figures for this year are not definite yet, as pilgrims continue to arrive from abroad, and the number of local ones is still uncertain. There are also always a number of illegal pilgrims without permits filtering through the various checkpoints leading to Mecca. By Tuesday, police had stopped 29,000 of those. The road between Mecca and the Red Sea port of Jeddah, which is the gateway for many pilgrims, is dotted with checkpoints to verify the documents of people travelling to the holy city. On Wednesday, Prince Nayef raised the spectre of a possible Al-Qaeda attack, but suggested it was unlikely. 'We cannot trust them. We do not rule out any attempt to disturb the security of the hajj,' he told reporters when asked about a possible threat by Al-Qaeda, which is growing strong in neighbouring Yemen, with prominent Saudi elements. 'We are ready for any act that might take place. God willing, nothing of that will happen, out of respect to this rite,' he said after a demonstration of skills by security forces and civil defence. Special troops dangled from helicopters, climbed moving ladders used in storming attacks, and acrobatically jumped from deployment vehicles, while special rapid- deployment squads and anti-riot vehicles were on parade. The head of Saudi General Security, General Saeed al-Qahtani, said 'these men are ready to face any situation when needed.' Saudi authorities also face the danger of fatal incidents during the pilgrimage. At Wednesday's parade, civil defence units brought out dozens of fire engines, ambulances, cranes that can lift up to 160 tonnes, smoke extractors and lifeboats, all of which have been used at some point in hajj incidents. The kingdom has been spending continuously on projects aimed at expanding the capacity of Mecca to accommodate the ever-increasing number of pilgrims. The most recent addition is the Mashair Railway, a light rail track linking the sites of Mina, Muzdalifah and Mount Arafat -- areas that see massive crowding. The train link will operate at 35 percent of its final capacity. It will replace 4,000 buses previously used, said Saeed al-Qurashi, head of the Hajj and Umra Committee at the Mecca Chamber of Commerce. The area has seen deadly stampedes in the past, mainly at the Jamarat bridge in Mina, where pilgrims rush while performing the symbolic rite of stoning the devil. In January 2006, 364 pilgrims were killed in a stampede at the entrance to the bridge, and 251 were trampled to death in 2004. In July 1990, 1,426 pilgrims were trampled or asphyxiated to death in a stampede in a tunnel, also in Mina. The deaths prompted authorities to dismantle the old bridge and replace it with a multi-level bridge. The last of the three levels will be opened this season, and pilgrim traffic has been made one-way to ensure a smooth flow. The new structure has numerous entrances and exits, while crowds are monitored with 30 cameras equipped with software that measures crowd density. Congestion is also a problem in the Grand Mosque which has a capacity of 700,000 in addition to 180,000 in its plaza. 'We are trying to adopt the Jamarat experience to the Grand Holy Mosque,' interior ministry spokesman General Mansur al-Turki said last month. Another new addition that cannot go unnoticed in Mecca is the world's largest clock, which sits atop a soaring skyscraper next to the Grand Mosque. The clock entered a three-month trial operation in August, but work is visibly going on to complete its four sides. The aim to establish a Mecca time for Muslims, like Greenwich Mean Time. The tower's height will reach 601 metres (1,983 feet), the official SPA news agency has said. On its website, Premiere Composite, which is responsible for cladding the top section, including a shimmering spire topped by a golden crescent moon, puts the planned height at 590 metres (1,947 feet). 