The island, about 150 km from Kolkata in West Bengal's 24 Parganas South district, now resembles a mini-India, as the devotees, including thousands of saints and seers jostled for space since the early hours braving the severe cold. A big batch of Naga sadhus have also descended for the fair.
The most auspicious period for the dip or 'shahi snan' (royal bath) started at 12.24 a.m. on Wednesday and would continue through the day.
The Gangasagar bath is considered auspicious by the Hindus, who gather here on Makar Sankranti for a holy dip and to pray by offering coconuts at the Kapil Muni temple. The devout believe that a single dip in the sacred water washes off the sins of one's lifetime.
Devotees from as far as Punjab, Jammu, Uttar Pradesh, Madhya Pradesh, as well as all other parts of the country, besides a substantial number of Hindus living in foreign geographies have congregated for the festival.
While an ocean of pilgrims gather here to take a dip every year, this time the attendance figure has scaled up to unprecedented levels since the Kumbh Mela, which is the other major religious festival that is accompanied by mass bathing, is not scheduled in 2020.
The district administration is expecting the turnout at Gangasagar to be close to five million by the end of the day.
"Over four million pilgrims have taken the holy dip till this morning," said an official.
Apart from more than five thousand police personnel, over one thousand CCTV cameras have been installed at various points of the fair premises. Two hundred and fifty drone cameras are also being used for surveillance, with a mega control room in the centre of the island coordinating the entire network.
The district administration has distributed one lakh QR code-based wrist bands to senior people and children to tackle the problem of missing persons. A real time crowd monitoring system has also been put in place through the application of information technology.
As part of the security measures, the coast guard has placed hovercraft, high-speed patrol vessels and interceptor boats along the coastline.
( With inputs from IANS )