Drone footage of the Maha Kumbh Mela Festival showed millions of devotees, shoulder to shoulder, arriving in the pre-dawn dark on Wednesday for the holy dip to mark the gathering's most auspicious day.
Video and photographs after the stampede showed bodies being taken away on stretchers and people sitting on the ground crying, while others stepped over a carpet of discarded belongings left by people as they tried to escape the stampede.
A Reuters witness saw several bodies as he followed dozens of ambulances rushing towards the river bank where the incident occurred.
A woman injured in the stampede is helped into an ambulance by the banks of the Sangam. (AP PHOTO)
Officials said an initial stampede occurred about 1am on Wednesday and was "not serious", but its cause was unclear.
However, witnesses said devotees trying to escape it were caught in another stampede at an exit. They then returned towards the pontoon bridges looking for another way out only to find it had been closed by authorities.
"I saw many people falling and getting walked on by the crowd...many children and women getting lost, crying for help," said Ravin, a devotee who gave only his first name and had travelled from the financial capital Mumbai for the festival.
A Rapid Action Force - a special unit called in during crisis situations - had been deployed to bring the situation under control and rescue efforts were underway, officials said.
Prime Minister Narendra Modi spoke to Uttar Pradesh Chief Minister Yogi Adityanath and called for "immediate support measures", reported news agency ANI, in which Reuters has a minority stake.
The world's largest congregation of humanity, the Hindu festival has already seen gigantic daily crowds, with nearly 148 million people attending since it began two weeks ago.
Authorities had expected a record 100 million people to throng the temporary township in Prayagraj on Wednesday for the holy dip, considered the most auspicious day due to a rare alignment of celestial bodies after 144 years.
Hindus believe that taking a dip at the confluence of three rivers - Ganga, Yamuna, and the mythical, invisible Saraswati - absolves them of their sins and confers salvation from the cycle of birth and death.
The "royal bath" was called off following the incident.
Hindu devotees take a holy dip by the banks of the Sangam during the Maha Kumbh festival. (AP PHOTO)
"All of our saints and seers were ready for the 'snan' (bath) when we were informed about this incident. That is why we have decided to call off our 'snan'," said ascetic Ravindra Puri.
Authorities had undertaken several measures to cater to the enormous crowd, including increased security and medical personnel, and special trains and buses. AI-software was also used to manage the crowds.
A similar stampede had broken out on the most auspicious day of the festival when it was last held in 2013, killing at least 36 pilgrims, mostly women.
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