ALLAHABAD, India (BBC) -- Millions of Hindus gathered in Allahabad in northern India as the Ardh Kumbh festival, which takes place once every six years, got under way.

Devotees braved chilly weather to bathe at Sangam, the confluence of three holy rivers, in a bid to wash away sins.

The festival is one of the world's largest gatherings.

Officials are expecting several million pilgrims on the first day alone.

A city of tents has been put up to house pilgrims and security is tight.

Groups of holy men - some smeared with ash, some dressed in bright saffron robes and some totally naked - held colorful processions on their way to the river bank to lead the dawn dip.

The festival got off to a smooth start, state official Pragyan Ram Mishra said, with more than three million people bathing in the first six hours of the day.

Pilgrims from across India are expected to attend the gathering "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, more than 50 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," state Home Secretary RM Srivastava said. ---------------------Auspicious days

Police say the number of people coming will differ from day to day, with much larger numbers bathing on particularly auspicious days.

Senior Superintendent of Police Rajeev Sabharwal told the BBC the largest crowd was expected on 19 January, while other big days were 14 January, 15 January and 23 January.

Allahabad is around 200 km southeast of Lucknow, the capital of Uttar Pradesh state, 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, broken every six years by the Ardh (half) Kumbh, the festival taking place at the moment.