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, 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 100 million people will pass through the temporary city that covers an area larger than Athens on a wide sandy river bank. That would make it larger even than previous festivals.
After a slow start, police chief Alok Sharma said 1.5 million people had gathered by 8 a.m. (0230 GMT) on Monday, with more on their way.
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, one incongruously wearing a suit jacket. At exactly five minutes past six (0035 GMT), 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.
"I wash away all my sins, from this life and before," said wandering ascetic Swami Shankranand Saraswati, 77, shivering naked in the cold. He said he gave up a career as a senior civil servant 40 years ago to become a holy man, travelled on foot and for decades ate only nuts and fruit.