Hindu worshippers began killing thousands of buffalo Tuesday in reputedly the world's biggest animal sacrifice, held every five years in a remote corner of Nepal, despite efforts to end the festival.

The Gadhimai Festival started in the early hours amid tight security, with the ceremonial slaughter of a goat, rat, chicken, pig and a pigeon. A local shaman then offered blood from five points of his body.

Some 200 butchers with sharpened swords and knives then walked into a walled arena bigger than a football field that held several thousand buffalo as excited pilgrims climbed trees to catch a glimpse.

"The sacrifices have begun today... We had tried not to support it but people have faith in the tradition and have come here with their offerings," Birendra Prasad Yadav from the festival organising committee told AFP.

Thousands of worshippers from Nepal and neighbouring India have spent days sleeping out in the open and offering prayers ahead of the event in Bariyarpur village, close to the Indian border.

"I believe in the goddess. My mother had asked her for the good health of my son," one of them said, holding a goat in his hand.

An estimated 200,000 animals ranging from goats to rats were butchered during the last two-day Gadhimai Festival in 2014, held in honour of the Hindu goddess of power.

Many were hopeful the centuries-old tradition would end after the temple authorities announced a ban in 2015 and Nepal's supreme court directed the government to discourage the bloodshed a year later.

But animal rights activists say both government agencies as well as temple committees have failed to implement these rulings.