An estimated two million Muslims are embarking on their annual hajj pilgrimage to the holy region of Mecca and Medina. This year's pilgrimage is taking place under tight security by Saudi officials.

Throngs of Muslims began their journey from Mecca into the small village of Mina Friday, to spend the day in prayer and meditation.

During the five-day hajj, the pilgrims will perform a series of rituals that replicate the path of the prophet Ibrahim 14 centuries ago, as described in the Koran.

The Saudi interior ministry says it has tightened security to guard against possible terrorist attack and to manage huge crowds.

On Thursday, Saudi forces apprehended several terrorist suspects 700 kilometers east of Riyadh, following a gun battle in which five Saudi agents and one civilian died.

Deadly incidents during hajj are not uncommon. More than 1400 people lost their lives in a stampede in 1990. A major fire swept through an encampment in 1997, taking 340 lives, and last year, 14 people were trampled to death on the third day of the ritual.

Rain has been forecast in the area over the next few days, raising concerns of mudslides, falling rocks and slippery conditions.

Every able-bodied Muslim is required to make the pilgrimage to Mecca at least once. The journey marks one of the five pillars of the Islamic faith.