SAN BARTOLOME DE PINARES, Spain - Thick smoke fills the cobbled streets of San Bartolome de Pinares as the sound of galloping hoofs edges ever closer to a bonfire crackling nearby.

Suddenly a horse and its rider burst through the flames as the mysterious age-old festival of Las Luminarias kicks off in this village nestled deep in snow-capped mountains near Madrid.

About 130 horses follow suit, ridden by young and old alike, with couples, parents and children taking part in the annual ritual village historians say is unique in Spain.

The event may have originated as a pagan ritual practiced by the Celts who lived in this area of Castilla y Leon around 1000 BC, says Salvador Saez, a 64-year-old retired teacher from the village who has researched the festival.

Fire - the great purifier - was thought to protect animals from diseases and give their riders fertility.

But that is mere conjecture. There are no written records and villagers have simply faithfully replicated the same annual routine for centuries.

"We've all asked the question of where it originates from, to our parents, our grandparents, and the response has always been 'since forever'," said Saez.

"No one has been able to give a concrete response."

So every year on the evening of Jan 16, the 600-strong village fills with smoke as residents light bonfires at regular intervals, warming cold onlookers but also forcing them to cover their mouths and noses.

Riders from the village and neighboring areas assemble at 9 pm and set off on a procession around the streets led by two walking locals playing the drums and dulzaina, a Spanish oboe-like instrument, clearing the fires downing wine or soft drinks.

After the ceremony, residents cook meat in the flames before partying the night away.

The next day, the bonfires are lit again to smoke up the streets in honor of Saint Anthony, protector of animals.

But activists have denounced an event they believe scares the horses.

In 2016, the PACMA animal rights party published footage showing several riders hitting their horses and one of the animals falling over.

Residents in San Bartolome de Pinares insist no horse is forced to go through the fire and add the animals belong to people who love and tend to them all year.

"There hasn't ever been an accident, and no burns," said organizer Jose Luis Escapez.

Agence France-presse