The Hindu religious event, held every five years for the past 260 years, has been held in Bariyarpur, about 160km south of the capital Kathmandu.
Hundreds of thousands of people attended the two-day event believing that the sacrifice of animals would please goddess Gadhimai and bring them luck.
But Manoj Gautam, an animal rights activist at the Jane Goodall Institute in Nepal, said that the religious aspect has seemed to have disappeared in recent years.
"There is no compassion. No spirituality. It's just a sport. It's a waste," CNN quoted Gautam as saying.
An estimated 500,000 goats, buffaloes, Pigeons and other animals were slaughtered in 2009, according to Humane Society International (HSI). That number dropped to around 30,000 in 2014.
In 2015, the event's organizer, Gadhimai Temple Trust, announced that it would no longer offer animals, according to CNN News 18. However, the massacre is still taking place this year.
In September, the Nepalese Supreme Court asked the government to plan to cancel the animal offering ceremony and pass the law to make it illegal, according to animal protection group Asia for Animal.
Gautam said his organization used drones to count the number of buffaloes before they were killed, discovering about 1,600-1,800 of them.
It is unclear how many other animals will be donated, when they are brought to the festival by local people. According to AFP, at least 3,000-6,500 buffaloes were killed on the first day of the festival.
