Hailed as the largest mass animal sacrifice, the Gadhimai mela (carnival), kicked off on Friday in the remote village of Bariyapur in Nepal.
The festival started at midnight amid tight security with the ceremonial killing of a goat, rat, chicken, pig and a pigeon.
Thousands of Hindu worshippers from Nepal and India participate in the festival where hundreds of animals are slaughtered to honour the goddess of power-Gandhimai.
Despite widespread opposition from animal rights activists, the once in five year event sees a steady flow of devotees.
Reports claim that many 'sword-wielding devotees' have poured into the village of Bariyapur near the Indian border for the world's largest abattoir that will be held for two days.
"It is very festive here, everyone is excited," said head priest Mangal Chaudhary at the slaughter site near a temple devoted to Hindu goddess Gadhimai. On the first day itself thousands of animal carcasses and severed heads have started piling up in a large field near the village where devotees are carrying out the sacrifices, eyewitnesses told AFP.
"It is very bloody... you can hear the animals moaning," said Rameshwor Mehta, 50, who was waiting to offer his prayers.
The sacrificial animals include buffaloes to rats. Chicken and goat meat are then distributed to the masses, while buffalo hides are auctioned after dumping their heads in a large pit.
The festival in 2009 reportedly saw about 300,000 animals slaughtered, however, this year the number is expected to cross 500,000, reports CNN-IBN.
Animal rights activists are dead against the festival. The campaign to ban the festival has the support of celebrities such as British actress Joanna Lumley and French movie legend Brigitte Bardot, who have requested Nepal's president to end the "cruel tradition".