Braving near-zero temperatures, the pilgrims swarmed into the sacred waters in Allahabad at the confluence of the revered Ganges and the Yamuna rivers for the Ardh Kumbh Mela, one of the largest gatherings of humanity on earth.

Sunday and Monday, two of the 45-day festival's six auspicious bathing days, mark Makar Sankranti, the transition of the sun into the tropic of Capricorn.

"This river is enchanting. You cannot stop. I dip into the water and it's a top-of-the-world feeling," said Mahendra Verma, a building contractor who was standing shivering in his underpants.

"At first one feels cold, but the feeling vanishes with one dip."

Pilgrims began bathing in the murky river waters early Sunday, hoping to break the endless cycle of reincarnation.

"Tens of thousands have already started taking baths (Sunday). In next two days over five million devotees are expected to take baths," said festival organiser P.N. Mishra.

Mishra said about 2.5 million pilgrims had already taken a dip since the festival began January 3.

Another bather, Kusma Devi, in her early sixties, said she had walked almost 25 kilometres from her village to reach Allahabad, some 400 kilometres from the Indian capital New Delhi.