Participants run ahead of bulls during the bull-run of the San Fermin festival in Pamplona, northern Spain, Monday. AFP-Yonhap

At least six people were injured during the second bull run in Pamplona in northern Spain, a spokeswoman for a hospital in the city told public broadcaster RTVE on Monday.

The injured runners, including a US citizen, mainly suffered from bruising from falls, despite the fact that the cobblestones of the old town were sprayed with anti-slip agents before the race, as they have been for years.

One of the bulls broke off part of one of his horns during the wild chase.

Hundreds of people will chase six massive bulls, some weighing over 600 kilograms, and several oxen through Pamplona's narrow alleyways into an arena until next Sunday as part of the annual San Fermín Festival, which dates to the 16th century.

The bulls are corralled into to Pamplona's bullring where they are then killed by matadors during bullfights.

While the San Fermín Festival also includes numerous concerts and processions, the bull runs and bullfights are the highlight of the festivities.

Dozens of runners, most of them young men, are injured every year as the race along the 875-metre course. There have been 16 fatalities since 1924, the last in 2009.

Animal rights activists call the bull runs "medieval cruelty," of a tradition that dates back to 1591.

Despite this, the festival continues to attract thousands of visitors from all over the world. (dpa)