Tens of millions of Hindus have gathered for a holy bath in India's sacred Ganges river for the most auspicious day of the world's largest religious festival.
Ash-smeared naked saints led the ritual bathing before dawn - which is said to cleanse pilgrims of their sins - with millions following them into the swirling river waters at the festival site in Allahabad in northern India.
The population of the city had swollen from its normal 1.2 million to about 40 million on Sunday morning, with about 20 million packed inside the vast sealed-off bathing area on the banks of the river, Ashok Sharma, a government spokesperson, said.
Amid the crush, the thousands of volunteers on duty and police were urging pilgrims to take one short dip and then leave the freezing waters to make space for others behind them.
"Aerial surveys by choppers, flying cameras and our estimates put the figure at around 20 million people taking a holy dip in the rivers," Sharma told AFP.
"Public address systems are asking people to leave the [steps] after bathing to avoid a crush."
The "Maha Kumbh Mela", which began last month and ends in March, takes place every 12 years in Allahabad. Smaller, similar events are held every three years in other locations around India.
The bathing takes place at an area called Sangam, the confluence of the rivers Ganges and Yamuna and a third mystical waterway called the Saraswati.
Devotees believe entering the mighty rivers cleanse them of sin and free them from the cycle of rebirth.
Assorted dreadlocked holymen, seers and self-proclaimed saints from all over the country have assembled for the spectacle that offers a rare glimpse of the dizzying range of Indian spiritualism.
Despite the hardships of waking early, plunging into the freezing and heavily polluted water and the crush of the crowds, pilgrims described being spiritually uplifted and amazed by the scale of the event.
Swapna Bhatia, an interior designer from New Delhi, called it "simply an out of the world experience".
Security 'in full swing'
More than 7,000 policemen have been deployed to oversee the Sunday bathing ritual, along with 30,000 volunteers, police say.
"The security is in full swing and our first concern as of now is the smooth exit of people after bathing as the number of devotees at Kumbh on this day has surpassed our expectations," police officer Ganganath Tripathi, who is overseeing security, told AFP.
The festival has its origins in Hindu mythology, which describes how a few drops of the nectar of immortality fell on the four places that host the festival - Allahabad, Nasik, Ujjain and Haridwar.
The "Mother Ganges" is worshipped as a god and is seen as the giver and taker of life.
"One dip in the river has the power to change life forever," said 65-year-old Malti Devi from London, who was taking part in the festivities for the first time.
Most devotees dunk their heads under the water, some drink it and others bottle it and take it home as gifts.
Management of the festival requires a monumental effort - and a budget of $290 million.
Thousands of buses and special trains were ferrying people to Allahabad where the heavily polluted Yamuna river flows into the Ganges. Despite its important role in Hinduism, the Ganges is tainted by industry and the settlements along its banks, which quickly turn the clear waters from the Himalayas into a murky, frothy brown downstream.