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.

Vast crowds of Hindu pilgrims in India bathed in sacred waters as the Kumbh Mela festival opened on Monday, with organisers expecting 400 million people -- the world's largest gathering of humanity -- to assemble over six weeks.India is the world's most populous nation, with 1.4 billion people, and so is used to large crowds.

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.