Tens of millions of Hindus are expected to plunge into India's sacred river Ganges to wash away their sins on the most auspicious day of the world's largest religious festival.
Organisers estimated some 30 million ash-smeared saints, devotees and visitors would take a dip in the swirling, freezing river at the festival in Allahabad in northern Uttar Pradesh state.
Sunday, when the water is considered to be the holiest and most auspicious, is "the most crucial day for the pilgrims and for police'', police officer Ajit Tyagi said.
The biggest concern is crowd control, he added.
"We have to make sure everyone is safe.''
More than 7000 policemen have been deployed to oversee the Sunday bathing ritual, along with 30,000 volunteers, he said.
Take a dip and move out of the water - those are the instructions we are giving,'' Manikant Mishra, an administrator at the Kumbh Mela, said.
Thousands more security personnel were guarding the sprawling site of makeshift tents.
Assorted dreadlocked holy-men, seers and self-proclaimed saints from all over the country have assembled for the colourful and chaotic spectacle that offers a rare glimpse of the dizzying range of Indian spiritualism.
Devotees believe entering the mighty river cleanses them of sin and frees them from the cycle of rebirth.
Many believe some three billion Hindu deities will also take a dip in the river to bless mankind on Sunday.
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.
The 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 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.
Most devotees dunk their heads under the water, some drink it and others bottle it and take it home as gifts.
Police said they would be guarding against stampedes - a frequent and deadly occurrence at Indian religious festivals.
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.