Naked Hindu ascetics and pilgrims are taking holy dips in sacred rivers in the first major bathing day of the Maha Kumbh festival, which is the largest religious congregation on Earth.

Authorities in India say more than 20 million people are expected to take part in the first major bathing day at the Hindu festival of Kumbh Mela (also known as the Mahakumbh), in the northern city of Prayagraj.

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