By Geeta Pandey BBC News, Allahabad India's Kumbh Mela festival begins
Several million people have been bathing at the confluence of the Ganges and Yamuna rivers at Allahabad in India, on the opening day of the Kumbh Mela festival.
At least 10 million pilgrims are set to do so by the end of the day.
The event, every 12 years, is billed as the biggest gathering on Earth. More than 100 million people are expected to attend the 55-day festival.
Hindus believe a festival dip will cleanse sins and help bring salvation.
In 2001, more than 40 million people gathered on the main bathing day of the festival, breaking a record for the biggest human gathering.
Sprint to waters The festival formally started at dawn on Monday. All roads leading to the Kumbh Mela grounds are packed with hundreds of thousands of pilgrims.
Continue reading the main story Kumbh Mela in numbers
- Visitors: 80-100 million
- Number of days: 55
- Area: 20 sq km (4,932 acres)
- Drinking water: 80 million litres
- Toilets: 35,000
- Doctors: 243
- Police: 30,000
There was a chill in the air as holy men sprinted into the waters in Allahabad, but the day dawned warmer than in recent weeks when a cold snap hit northern India.
Police estimated that by early afternoon about four million people had bathed.
For many at the festival, one of the most memorable spectacles of the day was when the Naga sadhus, or ascetics, sprinted into the river reciting religious chants, many clad only in marigold garlands.
The naked ash-smeared men arrived in a colourful procession and waded into the chilly waters of Sangam - the point at which the rivers converge.
The Kumbh Mela has its origins in Hindu mythology - many believe that when gods and demons fought over a pitcher of nectar, a few drops fell in the cities of Allahabad, Nasik, Ujjain and Haridwar - the four places where the Kumbh festival has been held for centuries.
Teams are managing crowds on the river bank - as soon as pilgrims finishing bathing, they are encouraged to move away and make space for other bathers.
"I have washed off my sins," Mandita Panna, a resident of Nepal and an early bather, said.
Allahabad has been preparing for the festival for months and a vast tented city has grown up around the river.
Fourteen temporary hospitals have been set up with 243 doctors deployed round-the-clock, and more than 40,000 toilets have been built for the pilgrims.
Police checkpoints have been set up on all roads leading to Allahabad and about 30,000 policemen and security officials have been deployed to provide security during the festival.
The main attraction at the festival is the sadhus, the Hindu holy men Tens of thousands of men, women and children have set up camp on the white sands of the river front.
On Sunday night, smoke could be seen rising from hundreds of small fires which people had built to cook dinner or keep warm.
One of the main attractions at the festival is the sadhus - Hindu holy men - who have been leading processions accompanied by elephants, camels, horses, chariots and music bands in recent days.
Health concerns The festival has prompted health concerns, however, with campaigners warning that the river waters are heavily polluted.
Most pilgrims drink a few drops of the Ganges water and many fill bottles to take home with them.
Authorities say they have taken steps to address the concerns.
Last week, companies along the banks of the Ganges and Yamuna were warned against discharging any pollutants into the waters.
Hindus believe a festival dip will cleanse sins and help bring salvation Reservoirs upstream have been ordered to discharge fresh water into the rivers ahead of the six big bathing days, and the festival authorities have declared the Kumbh Mela area a plastic-free zone.
The Kumbh Mela, which is costing the authorities 11.5bn rupees ($210m; £130m) to organise, is expected to generate business worth at least 120bn rupees, according to a report by India's Associated Chambers of Commerce and Industry (Assocham).
The report says that the festival is also expected to draw over a million foreign tourists.
