Vast crowds of Hindu pilgrims in India bathed in sacred waters Monday as the Kumbh Mela festival opened, with organisers expecting 400 million people -- the largest gathering of humanity.

Held every 12 years, 2025's Maha Kumbh mela has seen the construction of 400km of temporary roads, a 10,000-acre pop-up tent city, 150,000 toilets, three hospitals and more.

The festival that attracts over 400 million people takes place where Ganges, Yamuna and mythical Saraswati rivers meet.

Tens of thousands of Hindus seeking absolution of their sins immersed themselves on Monday in freezing waters at the confluence of sacred rivers, as India began a six-week festival expected to draw the world's largest gathering of humanity.

The Hindu festival of Kumbh Mela, the largest human gathering on the planet, began on Monday in northern India with a saffron tide of pilgrims flocking to the confluence of the sacred Ganges, Yamuna and Sarasvati rivers to bathe in the waters.

While the Kumbh Mela festival is ordinarly held every three years, rotating between several holy cities on the banks of these rivers, the Maha Kumbh Mela occurs only once every 12 years. Maha means “great”, and the event attracts the most devotees because it is considered the most auspicious and holy in the cycle.

The Maha Kumbh Mela, or the Great Pitcher Festival, will bring more than 400 million people to Prayagraj city in northern India over a period of six weeks starting Monday, making it the largest gathering of humanity in the world.