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 began on Monday its Maha Kumbh Mela, or Great Pitcher Festival, which is expected to draw more than 400 million visitors to the northern city of Prayagraj over six weeks, making for the largest gathering of humanity in the world.

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.

For pilgrims at the largest gathering of humanity, ritual bathing in India's holy rivers includes a key challenge -- finding your family and clothes after the chilly dip.

The Maha Kumbh festival — held every 12 years — started Monday, with more than 15 million pilgrims bathing in the holy rivers, according to officials.

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.

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.