More than a million pilgrims joined Islam's most important rite under a beating sun on Wednesday, as the hajj kicked off with the Saudi hosts scrambling to avoid last year's 1,000-plus deaths in sweltering heat.

With temperatures exceeding 40 degrees Celsius, robed pilgrims slowly circled the Kaaba, the black cube at the heart of Mecca's Grand Mosque which is Islam's holiest site.

Others arrived en masse in the sprawling tent city of Mina on Mecca's outskirts, where they will stay overnight before the hajj's high-point on Thursday - prayers on Mount Arafat, where the Prophet Mohammed is believed to have delivered his final sermon.

Authorities said about 1.5 million pilgrims had arrived in Saudi Arabia for the hajj, one of the five pillars of Islam that must be performed at least once by all Muslims with the means.

Authorities have ratcheted up heat protection measures such as extra shade to avoid a repeat of last year, when 1,301 people died as temperatures hit 51.8 degrees.

On Wednesday, pilgrims will perform the tawaf - walking seven times around the Kaaba, which Muslims pray towards each day.

Before entering Mecca, pilgrims must first enter a state of purity, called ihram, which requires special dress and behaviour.

Men don a seamless shroud-like white garment that emphasises unity among believers, regardless of their social status or nationality.

Women, in turn, wear loose dresses exposing just their faces and hands. (AFP)