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.

The Maha Kumbh is held once every 12 years at the Triveni Sangam -- the confluence of the rivers Ganga, Yamuna and the mythical Saraswati -- in Prayagraj.

Hindu devotees gather for a dip at the confluence of the Ganges, the Yamuna and the mythical Saraswati rivers on the first day of the 45-day-long Maha Kumbh festival in Prayagraj, India, Monday, Jan.

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.

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.

Over about the next six weeks, Hindu pilgrims will gather at the confluence of three sacred rivers where they will take part in elaborate rituals.

Drone video shows thousands of devotees bathing in sacred waters as the Maha Kumbh Mela, or the festival of the Sacred Pitcher, begins in Prayagraj, India. The celebration is expected to draw 400 million visitors.

Scores of naked Hindu ascetics smeared in holy ash charged into the water of holy rivers in northern India on Tuesday, dreadlocks flying, during the first 'royal bath' of the Maha Kumbh Mela, or Great Pitcher Festival.