More than 100 million people in the northern Indian city of Allahabad are preparing to bathe in holy waters as part of the world's largest religious gathering - the Kumbh Mela.
At least 15 million people are expected later at the confluence of the Ganges, Yamuna and the mythical, subterranean Saraswati rivers.
Officials are preparing for 120 million people during the 49-day festival.
It is billed as humanity's biggest gathering and can be seen from space.
Hindus believe that bathing at the confluence of the rivers - known as the Sangam - will cleanse their sins and help them attain salvation.
Kumbh Mela at a glance
- A pilgrimage in which Hindus gather at points along the Ganges, Yamuna and mythical Saraswati rivers.
- This year's event expects 120 million visitors over seven weeks, dwarfing last year's Hajj pilgrimage in Saudi Arabia which drew about 2.4 million.
- Astrology determines most aspects of the festival, including its date, duration and location.
- The most recent full Kumbh, held in 2013 in Allahabad, was also a Maha (or great) Kumbh, which happen every 144 years. It attracted an estimated 100 million visitors.
- This year, 15 lost-and-found camps have been set up. These computerised centres are interconnected and their announcements will be heard across the mela grounds. Details will also be uploaded on Facebook and Twitter to help trace the missing.
The biggest draw at the festival are the Naga sadhus - the naked ash-smeared ascetics who arrive in massive colourful processions.
At the last Kumbh in 2013, female ascetics were allowed to bathe at the Sangam for the first time. This time, hundreds of transgender people will be participating.
More than a million foreign pilgrims will also take part in the festival, senior administration official Rajeev Rai told the BBC.
He and other organisers have been preparing for more than a year for the event, which dwarfs the annual Hajj pilgrimage to Islam's holiest sites in Saudi Arabia.
"Last minute preparations are on. All the religious sects have been allocated time for their processions and bathing rituals," Mr Rai said.
"We have devised a traffic plan to ensure there's no overcrowding. The mela area is open only to pedestrians,"
The mela (meaning "fair" in Hindi) has been held in Allahabad for centuries now, but it has grown into a mega event in the past two decades.
This year the gathering will be particularly huge and many believe India's Hindu nationalist government has organised it with an eye on key general elections due in the summer.
Massive billboards of Prime Minister Narendra Modi dot Allahabad city and the mela ground. Huge cardboard cut-outs have been placed strategically at the bathing areas.
A temporary tent city, spread over 32 sq km has been set up to accommodate the masses, complete with hundreds of kilometres of new roads. Hospitals, banks and fire services have been set up just for the festival, along with 120,000 toilets.
Hundreds of new train services are running to and from Allahabad to tackle the rush of pilgrims and more than 30,000 police and paramilitaries have been deployed to provide security and manage the crowds.
In the run up to the festival, religious sects have held daily processions marked by much pomp and show.
At one such procession on Sunday night, there were elephants, camels and horses. Brass bands and drummers played, as religious leaders sitting atop several vehicles threw marigold flowers to thousands of devotees.
On Monday - a day before the official start of the festival - tens of thousands of pilgrims bathed at the Sangam. Some then lit clay lamps and floated them along with flowers in the Ganges.
The atmosphere at the mela is festive, and the authorities have announced a calendar of music and dance performances. But there's plenty of impromptu entertainment taking place by the roadside, with children performing rope tricks and shows by drummers and ballad singers.
Most pilgrims, however, say they are here to "answer the call of Mother Ganges".
"We believe that bathing here will destroy our sins," farmer Pramod Sharma said.
"The waters here have regenerative properties. Bathing here can cure your ailments. It also removes obstacles from your way," Shahbji Raja said.