It’s late evening in the city of Prayagraj and the roadside vendors selling snacks are packing up their wares to shut for the night.

Nearly 15 million Hindus, six times the number expected, took a dip in freezing waters seeking absolution of their sins on Monday, the first day of India's festival that could draw the world's largest single gathering of humanity.

India's Maha Kumbh Mela, or Great Pitcher Festival, is set to be the world's largest gathering of humanity, with 400 million visitors expected at the six-week event that began this week in the northern city of Prayagraj.