At least 18 million people have bathed so far in the river Ganges in northern India on the main day of the Ardh Kumbh mela, festival organisers say.
Thousands of Hindu holy men began the plunge near the town of Allahabad at dawn. Crowds are still arriving and bathing will continue into the night.
It is thought to be the largest gathering of humanity on the planet.
Pilgrims believe that bathing at the confluence of three of Hinduism’s holiest rivers washes away their sins.
The mass bathing is taking place at Sangam, where the Ganges, Yamuna and mythical Saraswati rivers meet.
The new moon night, or Mauni Amavasya, is celebrated on Friday, making it the most auspicious day in the six-week-long festival.
Ash-smeared Naga sadhus ran into the water at the start of the day wearing nothing apart from marigold garlands.
The mela’s chief organiser, RN Tripathi, said more than 18 million people had taken a holy dip by 1700 hours local time (1130 GMT).
Another two million people are expected to have followed suit by the end of the day.
The Mauni Amavasya is a day when sun, moon, Venus and Mercury are in the zodiac of Capricorn, a rare but perfect alignment of planets, devotees believe.
One pilgrim, Shri Mahant Vindgiri, explained why the day was so special.
“The planetary alignment is such that sun rays, when they fall on the Ganges, turn the river water into nectar. So bathing here today is equivalent to drinking nectar,” he says.
Some say all amavasyas – or new moon nights – are auspicious for bathing in the Ganges, but Mauni (silent) Amavasya is also special as many pilgrims do not speak until they have had their bath.
“It is believed that on this day taking a bath without breaking your silence will bring the benefit of performing millions of yagnas [ritual offerings],” says Gulab Singh Yadav, a resident of Allahabad.
The BBC’s Geeta Pandey, who is at the festival, says all roads leading to the site are teeming with pilgrims and more keep arriving.
Men with duvets and blankets slung across their shoulders, women carrying or holding small children by hand, saffron-robed sadhus – or holy men – are all at the site.
Many have trekked for miles.
No traffic is allowed on the roads leading up to the river and thousands of policemen and paramilitary troops have been deployed to ensure the orderly movement of people.
Under the alert eyes of security officials, hundreds of vendors sell marigold flowers and sweets, sacred thread and cans to take away the Ganges water, our correspondent says.
The Mela area, spread out over several square kilometres, has been turned into a tent city with thousands of tents – big and small – covering every bit of ground.
The festival began on 3 January and concludes on 16 February. Organisers expect about 60 million people to attend over the six weeks.