Millions of people have taken a holy bath at the Hindu festival of Kumbh Mela (also known as Mahakumbh) ? described as humanity?s biggest gathering ? in northern India?s Prayagraj city on Monday.

The event - held once every 12 years ? starts on Monday and over the next six weeks, the devout will bathe at Sangam ? the confluence of India?s most sacred Ganges river with the Yamuna river and the mythical Saraswati.

Hindus believe that taking a dip in the sacred river will cleanse them of sins, purify their soul and liberate them from the cycle of birth and death ? as the ultimate goal of Hinduism is salvation.

About 400 million pilgrims are expected to attend the 45-day spectacle, which is so large it can be seen from space.

Authorities said that on Monday, until 4pm local time, 16 million people had taken a bath. On Tuesday, numbers are expected to exceed 20 million, and the spectacle will be special as it will see ash-smeared naked Hindu holy men with matted dreadlocks, known as Naga sadhus, take a dip at dawn.

But authorities are racing against time to get the city ready to host millions who will continue to pour in throughout the festival.

To accommodate the pilgrims and tourists, a vast tent city, sread over 4,000 hectares, has been set up on the banks of the river.

But on Sunday, just hours before proceedings were due to begin, many parts of the sprawling grounds in Prayagraj still appeared to be a work in progress.

Some of the camps set up by saints and other worshippers had no water and intermittent power supplies.

Thousands of toilet cubicles were still yet to be set up and many already installed were unusable because of missing water connections.

Administration official Vivek Chaturvedi told the BBC that organisers were hampered by the fact that this year the monsoon waters took longer to recede which narrowed the window for construction activities.

But, he insisted, ?preparations are almost complete and all systems will be in place to welcome the visitors?.

?We have laid 650km (403 miles) of temporary roads and set up tens of thousands of tents and toilets. More than 100,000 people, including over 40,000 police and security officials, are working round-the-clock to make it a success,? Mr Chaturvedi said.

What is Kumbh Mela?

The festival, which concludes on 26 February, has been recognised as an Intangible Heritage of Humanity by the United Nations agency Unesco.

Its origin is rooted in a mythological story about a fight between the gods and demons over a Kumbh (a pitcher) of nectar that emerged during the churning of the ocean.

As the two sides fought over the pot of elixir that promised them immortality, a few drops spilled over and fell in four cities ? Prayagraj, Haridwar, Ujjain and Nasik.

As the fight went on for 12 celestial years ? each equal to 12 years on Earth ? Kumbh Mela festival is held every 12 years in the four cities. An ardh or a half Kumbh is organised halfway between two festivals.

The mela is organised in all the four cities, but the biggest festivals, where previous attendance records are broken, are always held in Prayagraj.

Hindu seer Mahant Ravindra Puri said the festival this time round was ?extra special? and described it as ?a Maha [great] Kumbh?.

?That?s because the current alignment of planets and stars is identical to what existed at the moment of the spill,? he told the BBC.

?Such perfection is being observed after 12 Kumbh festivals or 144 years,? he said.

For festival-goers, a major attraction is the presence of naked Naga sadhus, or ascetics, and it is a spectacle to watch as they hurl themselves into the icy waters.

But for the devout, it holds special significance ? they believe that the waters get imbued with the purity of the saints? thoughts and deeds.

Over the weekend, groups of holy men arrived at the mela grounds in large noisy processions.

One group of ash-smeared men, some naked and some dressed in just loin cloth or marigold garland draped around their necks, marched in holding tridents, swords and small two-headed drums.

Another group had its leaders on chariots escorted to their campsite in a large procession with a music band, dancers, horses and camels.

What are the big bathing days?

The bathing dates and auspicious times are decided by astrologers, based on the alignment of specific planets and constellations.

There are six particularly auspicious days to bathe this time:

13 January: Paush Purnima

14 January: Makar Sankranti

29 January: Mauni Amavasya

3 February: Basant Panchami

12 February: Magh Purnima

26 February: Maha Shivaratri

Three of these ? 14 and 29 January, and 3 February ? have been designated as Shahi Snan (or the royal bath) days when the Naga sadhus will bathe.

The largest gathering Is expected on 29 January when 50 to 60 million worshippers are expected to take to the waters.

Away from the riverside, the city of Prayagraj has been decked up for the mega event.

Officials said about 200 roads had been widened and a fresh coat of paint had been applied to facades leading to Sangam, while walls have been decorated with colourful paintings and murals depicting stories from Hindu mythological texts.

Tens of thousands of pilgrims, including many from foreign countries, have already reached the city.

Sebastian Diago, visiting as part of a 90-member group from Argentina, said he made the journey to ?experience the devotion first hand?.

?I felt the pull of the Ganges so I came,? he said.

?I will bathe in the river because I feel the need to connect with the Ganges.?

The Indian government said it was spending 70bn rupees ($812m; ?665m) on organising the festival and according to local media reports, the state government will earn a revenue of 250bn rupees ($2.9bn; ?2.3bn).

Saints and leaders of big campsites said they understood the complexities of organising a festival on such a large scale, but some pilgrims complained about a lack of facilities.

Baba Amarnathji, a 60-year-old saffron-robed monk, showed the BBC a small tent he had set up for himself with cloth and plastic sheets draped over three bamboo poles.

On earlier occasions, he said, he could sleep for free in tents set up by the administration, but this time there was no such facility.

?The police try to chase me away from here. But where will I go? Everyone says this festival is meant for sadhus like me, but I see that all the arrangements are being made for tourists.? (BBC)

Update news