You can hear them before you see them - the rumble of hundreds of hooves thumping on the road leading to the town of Cuellar in Castile and Leon, Spain.

During the weekend, fighting bulls were herded through pine forests and fields to the town by men on horseback. Many of the herders carried lances to protect their horses from the bulls.

This is part of the Fiesta de Cuellar, a tradition that goes back to 1499 and is recognised as one of the oldest of its kind in Spain.

The bulls are meant for the encierro or bull runs, a practice where people try to outrun the animals after they have been released into town streets. The first encierro of the year took place on Sunday.

Some say this event originates from the need to transport bulls from fields where they were bred to the marketplaces in town. People would jump into the midst of a herd to show off how brave they were, starting a custom that survives to this day.

Sunday's event passed without incident, except for one runner who suffered some slight bruising.