Three million people from all over the world are expected in Saudi Arabia’s holy cities of Mecca and Medina for the annual Islamic hajj pilgrimage, which starts on Tuesday amid tight security.

The Saudi authorities are deploying 100,000 security personnel, including members of the counter-terrorism and the emergency civil defence units, and 5,000 cctv cameras.

As well as fears of a possible terrorist attack, there are massive logistical crowd and traffic control issues in handling the world’s biggest pilgrimage, which lasts about five days.

“We always concentrate on hajj considering that a threat might exist,” Major General Mansour al-Turki told Associated Press. “We’ve been targeted by terrorism for years now and we know that we are a target for terrorist groups.”

This year’s hajj follows several earlier attacks, claimed by Isis-affiliated groups, in which more than 60 people were killed. In the most recent, 15 died in an attack on a mosque inside a police compound in Abha, close to the Yemen border, last month.

Pilgrims have been undeterred by the collapse of a construction crane in Mecca earlier this month, which killed more than 100 people and injured at least 200. An investigation has been launched into the cause of the collapse amid claims the proper safeguards are ignored in the race to complete developments surrounding the Grand Mosque. Among the projects under construction is a 10,000-bed hotel.

Last week, more than 1000 pilgrims were evacuated from a hotel in Mecca in the early hours of the morning after a fire broke out.

There have been stampedes during the hajj on at least six occasions, killing hundreds of pilgrims. In 1990, more than 1,400 died in a stampede inside a tunnel.

It is a religious duty for able-bodied Muslims to complete the hajj – one of the five pillars of Islam – at least once. The rituals involved in the pilgrimage are intended to cleanse the soul and promote the bonds of brotherhood and sisterhood within Islam. The Guardian