By Geeta Pandey
BBC News, Allahabad
Millions of Hindus have gathered in Allahabad in northern India for the Ardh Kumbh, a half-size version of the better known Kumbh Mela festival.
Hindus believe a dip at Sangam can wash away sins
The festival, which began at dawn, will go on for a month and a half.
Thousands of tents have been put up and 20,000 policemen have been deployed at the mela grounds.
Hindus believe that bathing at Sangam, the confluence of three holy rivers - Ganges, Yamuna and the mythical Saraswati - can wash away their sins.
Groups of holy men - some smeared with ash, some dressed in bright saffron robes and some totally naked - arrived in Allahabad and held colourful processions on their way to the river bank.
Hordes of people have been pouring into the town from across the country and groups of foreign pilgrims have also been arriving.
On the first day of the festival on Wednesday, officials say they expect six to eight million pilgrims to take a dip at Sangam.
"The number of people coming will differ from day to day," senior superintendent of police, Rajeev Sabharwal, told the BBC.
"We are expecting the maximum crowd on 19 January - around 40 million people are expected to take a bath on that one single day.
"Apart from that, we are expecting that 20 million pilgrims will turn up on each of the other three big bathing days of 14 January, 15 January and 23 January," he said.
Mr Sabharwal, who is in charge of security arrangements, has been working for months to ensure that everything passes off peacefully.
Millions of people are expected to attend the festival
With such large crowds gathering, the biggest fear is a stampede.
In 1954, more than 800 pilgrims were killed in a stampede during the Kumbh festival in Allahabad and officials say they are doing everything to ensure no such incidents take place this time.
"Our primary responsibility is to ensure that pilgrims have a safe bathing. We have to regulate their movement to ensure they have got enough space. So we stagger the crowds, allow different groups to bathe at different times," says Mr Sabharwal.
He says close to 20,000 policemen and central paramilitary soldiers are being deployed to ensure safety and security at the mela grounds.
The police are being helped in their work by dozens of non-governmental groups who are helping with traffic and crowd management, and also helping those who get separated from their families.
According to Hindu mythology, gods and demons fought a celestial war over a pitcher of divine nectar. Allahabad is one of the four towns where drops of nectar fell during the battle.
Groups of holy men have arrived in Allahabad
The war lasted 12 divine days (which is equal to 12 human years) and that led to the celebration of the Maha Kumbh - the most auspicious gathering held every 12 years.
The half kumbh is celebrated every six years and is also considered auspicious by the devout.
But many say these festivals have now lost their religious sanctity and a large number of visitors come here for reasons other than divine salvation.
"Earlier, only 8-10,000 people came for the mela. But now it's become like a picnic and people come here to have fun," says 80-year-old Raja Ram Tiwari who has been setting up a "lost-and-found" camp at the mela grounds for six decades now.
Also, many long-time residents of Allahabad say the river water is getting increasingly polluted and is no longer fit for a dip.
But mela authorities say they have taken steps to ensure that the river water is free of pollution.
Mela organiser Pragyan Ram Mishra told the BBC: "We are taking all measures to ensure that the river is kept clean. Pollutants coming into the Ganges from at least two of the major drains has been stopped and the water quality is good.
"All those who want to come here for the 2007 Ardh Kumbh can come here without any concerns. They will get clean water for bathing and drinking. That is my assurance," he says.
Mr Mishra says the Uttar Pradesh state government has sanctioned $38 million for the festival.