More than 30 million people are expected to plunge into the waters at the confluence of India's Ganges and Yamuna rivers in the northern city of Allahabad Sunday.
It is the most auspicious day of the Hindu festival, Kumbh Mela, which is held every 12 years.
Devotees believe entering the mighty rivers cleanses them of sin and frees them of the cycle of birth and rebirth.
Some 100 million pilgrims are expected to take a dip in the rivers during the 55-day festival that began last month.
The festival is believed to date back 2000 years. According to Hindu mythology, when gods and demons fought over a pitcher of nectar that gives immortality, some drops fell on four towns, including Allahabad. These towns host the festival in rotation.
The one at Allahabad is believed to be the holiest and surpasses the others in its sheer scale and size.