Several million Hindus have taken part in day one of the Ardh Kumbh festival in northern India, officials say.
Devotees braved the cold to bathe at Sangam, the confluence of the Ganges and Yamuna holy rivers near Allahabad.
Rituals began at daybreak - Hindus believe the holy dip washes away sins. The festival, held every 12 years, is one of the world's largest gatherings.
A city of tents will house an expected 60 million pilgrims over the next six weeks and security is tight.
Groups of holy men - some smeared with ash, some dressed in bright saffron robes and some totally naked - held colourful processions on their way to the river bank to lead the dawn dip.
Hundreds of thousands of men, women and children then jumped into the water as the festival began, despite cold winds and fog.
"Initially, I felt some cold. But one dip and the cold was gone," Ram Vir Upadhaya, a retired government official, told the Associated Press news agency.
The festival got off to a smooth start, state official Pragyan Ram Mishra said, with more than three million people estimated to have bathed in the first six hours of the day.
Millions of pilgrims have travelled to Allahabad
"Controlling crowds will be our top priority as we are expecting up to 10 million people here on Wednesday itself," he told the French news agency AFP.
Over the 45-day period, an estimated 60 million pilgrims are expected from India and overseas.
A huge tented city capable of housing two million devotees has been erected close by, as well as 25,000 toilets to cope with the influx.
Around 20,000 police have been deployed to control crowds, amid fears of stampedes.
"Security has been tightened around the mela [festival] grounds and our policemen would mill around in plain clothes and some will be deployed dressed as sadhus [holy men] at strategic points," said RM Srivastava, home secretary of Uttar Pradesh state where the event is taking place.
Police say the number of people coming will differ from day to day, with much larger numbers bathing on particularly auspicious days.
Ardh Kumbh Mela, India: 60 million expected
New Year's Eve Times Square, New York: 1 million
Hajj, Mecca, Saudi Arabia: 3 million
Population of UK: 60 million
Senior superintendent of police Rajeev Sabharwal told the BBC the largest crowd was expected on 19 January. The other big festival days are 3 January, 15 January, 23 January, 2 February and 16 February.
Allahabad is about 200km (125 miles) south-east of Lucknow, the Uttar Pradesh capital, and the Ganges, Yamuna and the mythical Saraswati rivers converge there.
According to Hindu mythology, gods and demons fought a celestial war over a pitcher (called a kumbh) of divine nectar. Allahabad is one of the four towns where drops of nectar fell during the battle.
The war lasted 12 divine days and so a full Kumbh festival is held every 12 years, the last being in 2001.
The gap is broken mid-way by the Ardh Kumbh Mela, or Half Pitcher Festival, which will go on until halfway through February.
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