Gadhimai Mela, the world's biggest animal fair held once every five years, began at Bariyarpur in Bara district of Nepal bordering Bihar on Friday.
Thousands of buffaloes, mostly calves, were sacrificed at an enclosed field close to the temple of Goddess Gadhimai, a Hindu deity, believed by devotees to grant wishes if they sacrifice animals and birds.
But unlike previous occasions, the total number of sacrifices is expected to be much lesser this time due to efforts by animal rights activists and a ban on bringing animals to the fair from India.
Sacrifices at the fair began on Thursday night with 'pancha bali'---where five animals—a pig, pigeons, duck, hen and rat were offered to the goddess by the priest of the temple.
The sacrifice of buffaloes began early on Friday morning where hundreds of locals armed with swords, machetes and khukuris started the mass killing.
Hundreds of thousands of devotees from all over Nepal and from neighbouring India are converging at the site and many have brought animals are birds along to offer the goddess.
"Over 1300 police personnel have been deployed near the temple to manage the smooth flow of devotees. Routine checking is also being conducted," said a local police officer.
The ritual continues for two days and on the second day goats are sacrificed. Devotees and residents of villages near the temple sacrifice rats, pigeons, ducks, hens and goats themselves.
Nearly 25,000 buffaloes were killed during the 2009 fair, but there is no data on the total number of animals and birds sacrificed. Some estimate it to be around 500,000.
This year due to efforts of animal rights activists to spread awareness and make people desist from offering animals and birds to appease the goddess, the figure is expected to be much less.
"Besides faith there is a network of vested interests who want to promote the killings to earn profits," Indian activist Swami Agnivesh who is camping near to temple to stop the sacrifices told a Nepali news channel.
Last month India's Supreme Court directed the government to enforce a ban on illegal entry of animals to Nepal especially for the Gadhimai Mela.
Usually nearly 70% of the buffaloes and goats sacrificed at the fair come from India. But enforcement of the ban has led to drop in figures.
It was estimated that nearly 5,000 buffaloes were sacrificed within few hours on Friday morning. The killings will continue on Saturday.
A news report says nearly 125,000 goats have been sold in local markets near the temple in the past three months. They will be sacrificed on Saturday.