Nearly 70 million Hindus are expected to bathe in freezing cold water at the confluence of three major rivers in north India as part of Ardh Kumbh Mela, a pilgrimage intended to wash away earthly sins.
The makeshift campsite of 50,000 tents and 25,000 lavatories, provided by the authorities for one of the largest gatherings of people in the world, covers 30 square miles on the banks of the Ganges.
Nearly 10 million Hindus, led by ash-smeared holy men, are to bathe at the confluence of Jamuna, Ganges and Saraswati rivers today - the first day of the festival. Winds and morning fog are expected to bring the temperature down to about 6C.
Ardh Kumbh Mela, or Half Pitcher Festival, is held every six years at Allahabad, about 120 miles south-east of Lucknow, the capital of Uttar Pradesh state.
Nearly 50,000 policemen will be on duty and providing food, water and shelter are other key worries. Authorities have spent more than 7.5 billion rupees (£86m) to provide infrastructure.
According to Hindu mythology, gods and demons fought a celestial war, spilling nectar at Allahabad in a pitcher or Kumbh.
Hindus believe that bathing in the Ganges during the festivals washes away their sins and ends the process of reincarnation.
Hindu religious leaders have started arriving in the area, some entering the river banks in chariots pulled by white horses, others carried by devotees on sedan chairs.Reuse content