Over one million elated Hindu holy men and pilgrims took a plunge in India's sacred Ganges river to wash away lifetimes of sins, in a raucous start to an ever-growing religious gathering that is already the world's largest.

Once every 12 years, tens of millions of pilgrims stream to the small northern city of Allahabad from across India for the Maha Kumbh Mela, or Grand Pitcher Festival.

The festival takes place at the point where the Ganges and Yamuna rivers meet with a third, mythical river.

Officials believe that over the next two months as many as 100m people will pass through the temporary city on a wide sandy river bank.

That would make it larger even than previous festivals.

After a slow start, police chief Alok Sharma said the crowds had swelled to over 1.5m people.

Two dreadlocked men riding horses emerged from thick camp smoke before dawn, followed by a crowd of ash-smeared and naked holy men, or sadhus.

At exactly 6.05am (12.35am Irish time), they yelled and dashed dancing into the river.

That the ancient festival grows in size each time it is held partly reflects India's expanding population, but is also seen as evidence that spiritual life is thriving alongside the new-found affluence of a growing middle class.

The ritual "Royal Bath" was timed to match an auspicious planetary alignment, when believers say spiritual energy flows to earth.

The festival has its roots in a Hindu tradition that says the god Vishnu wrested from demons a golden pot containing the nectar of immortality.

In a 12-day fight for possession, four drops fell to earth, in the cities of Allahabad, Haridwar, Ujain and Nasik. Every three years a Kumbh Mela is held at one of these spots, with the festival at Allahabad the holiest of them all.

More than 2,000 years old, the festival is a meeting point for the Hindu sadhus, some who live in forests or Himalayan caves, and who belong to dozens of inter-related congregations.

The sects have their own administration and elect leaders, but are also known for violent clashes with each other.

Some naked, some wrapped in saffron or leopard-print cloth and smoking cannabis pipes, the holy men hold court by fire pits in sprawling camps decorated with coloured neon lights, where they are visited by pilgrims who proffer alms and get blessings.

To cope with the flow of people, authorities in the state of Uttar Pradesh have installed 35,000 toilets, laid 550km of water pipes and 155km of temporary roads at the riverbank site.