Hindu worshippers have begun beheading thousands of buffalo in Nepal as part of what is thought to be the world’s largest animal sacrifice event.

The Gadhimai festival started in the early hours of Tuesday morning as a priest killed a goat, rat, chicken, pig and pigeon and then offered blood from his own body.

Around 200 butchers wielding curved kukri knives then descended on a walled temple arena containing an estimated 3,500 buffalo to behead the animals.

Campaigners for animal rights charity Humane Society International described seeing baby buffaloes bellowing as they watched their mothers being decapitated.

They said others collapsed from exhaustion, sickness and stress as worshippers attempted to drag them to their deaths.

Thousands of people from Nepal and neighbouring India travelled to Bariyarpur village to witness the festival, which occurs every five years.

A Hindu devotee prepares to slaughter a buffalo (AFP via Getty Images)

The sacrifices went ahead despite temple authorities announcing a ban in 2015 and Nepal’s supreme court directing the government to reduce animal sacrifice a year later.

Alokparna Sengupta, managing director of Humane Society International in India, said being at the festival was “one of the most depressing and challenging experiences of my life”.

“The suffering of these animals is so upsetting, they have endured exhausting journeys to get here and are paraded in front of a baying crowd as all around them they witness other animals being decapitated one by one,” she said.

“Buffalo calves look on in bewilderment as their mothers are slaughtered in front of them. The hysteria and apparent jubilation at seeing confused and frightened animals being slaughtered was very disturbing.”

In the last week the Indian border force and campaigners seized hundreds of baby goats, pigeons and buffaloes at the border.

“We may not have a bloodless Gadhimai this time, but we are determined that one day we will see an end to this gruesome spectacle,” Ms Sengupta added.

Mimi Bekhechi, Peta’s director of international programmes, said: “Nepali officials promised – but have failed – to protect thousands of petrified animals from being dragged and transported to a violent mass killing that starts today.

“The government must act forcefully to prevent this bloodbath from taking place again or feel the impact of a tourist boycott.”

She added: “Devotees are bringing universal condemnation to a wonderful religion – which needn’t be based on fear and cruelty – and just as widow-burning and human-baby sacrifice have ended, so must the horror of an animal-slaughter festival.

“Gruesome images of animals looking up pitifully and struggling as they’re hacked into pieces ruin Nepal’s reputation, and we urge all of its compassionate citizens to join the international clamour for an end to such barbarity.”

In 2009, at the height of the festival, around 500,000 buffaloes, goats, pigeons and other animals were killed. After protests in the intervening years 30,000 were killed in 2014.

Devotees believe the ritual slaughter brings good luck and encourage Gadhimai, the Hindu goddess of power, to answer their wishes.​