Thick smoke fills the cobbled streets of San Bartolome de Pinares as the clip-clop of galloping hooves 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.
Some 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 practised by the Celts who lived in this area of Castilla y Leon more than 1,000 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.
The ritual could have been "christianised" later by the Catholic Church, he adds.
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'," says Saez.
No one has been able to give a concrete response.
So every year on the evening of January 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 neighbouring areas assemble at 9 pm (2000 GMT) 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 one by one and downing local wine or soft drinks in the process.
The final stretch on the main Road of the Virgin sees the more fool-hardy careering up, their horses leaping over the flames or passing next to them, narrowly missing -- and at times hitting -- people in the crowds pressed together to watch.
After the ceremony, residents cook meat on the blazes before partying the night away.
The next day, the bonfires are lit again to smoke up the streets in honour of Saint Anthony, protector of animals.
After almost disappearing in the 1960s as people left the village for the lure of cities, Las Luminarias was regenerated by a group of youths keen to preserve the ritual.
The difference now is that the horses used to jump the fire are no longer farming the fields, but are prized possessions of keen riders.
Before nightfall, Diego Martin prepares his horse Dandy, a handsome brown Selle Francais who has never before participated in the ritual.
As he braids his mane and covers his tail with a cloth to protect them from the flames, Martin says he isn't worried.
If he wants to go through, he will. But if he doesn't like it and he gets scared, he won't," says the 38-year-old, whose parents are from the village but who grew up in Madrid.
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.
On Tuesday evening, a few riders were seen with sticks but a large majority went without, many throwing their hands up in the air while they jumped to show they were not hitting their animals.
One horse fell down near a bonfire, and promptly got up again.
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 the them all year, and would not put them in any danger.
There hasn't ever been an accident, and no burns," says Jose Luis Escapez, 57, one of the organisers of this year's event.