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.

The Kumbh Mela, a revered Hindu festival, occurs every three years in four sacred Indian locations: Haridwar, Prayagraj, Ujjain, and Nashik. Every 12 years, each site hosts the grand Maha Kumbh. This year's event will be held in Prayagraj.