Some 60 million Hindus are expected to take part in the Ardh Kumbh Mela festival which has got under way in Allahabad in northern India.
The pilgrims braved the early-morning cold to arrive at the confluence of the Ganges, Yamuna and the mythical Saraswati rivers in a bid to wash away their sins.
The main period of bathing is on six auspicious days, depending on the alignment of stars.
On the first of these days, 3 January, nearly 10 million people, led by ash-smeared and naked holy men, are expected to take a dip.
One of the pilgrims performs a ritual by pouring milk in the river after taking a dip despite cold winds and fog.
The Hindu faithful believe that bathing during the festival not only washes away their sins, but also ends the process of reincarnation.
Although the Ardh Kumbh Mela is a "half-size" version of the better-known Kumbh Mela, a huge logistical operation is still required.
Logistics to provide food, water and shelter to the milling crowds are key worries for officials throughout the Ardh Kumbh Mela, which takes place every 12 years.
Some 50,000 tents and 25,000 makeshift toilets cover an 80 sq-km (30 sq-mile) area on the banks of the Ganges.
However, some devotees have complained about the lack of proper sanitary facilities.