Millions of Muslims have stoned pillars representing the devil in a symbolic rejection of temptation on the second day of their annual hajj pilgrimage.

Crowds on Sunday filed past the three pillars, which now resemble curved walls, in a four-level sprawling concrete structure built to expedite the flow of pilgrims, energetically casting pebbles at the largest one. Saudi authorities estimate that 2.5 million pilgrims have joined the hajj this year.

The ritual will be repeated for two more days, with participants eventually casting stones at all three pillars.

Sunday also marks the start of Eid al-Adha, a festival in remembrance of Abraham's near-sacrifice of his son.

The hajj is one of Islam's five pillars and is required of every able-bodied Muslim once in their lifetime.