Three men have been badly gored and several others injured in the first day of the Running of the Bulls in Pamplona.
The festival attracts runners from all over the world, and involves people running ahead of six fighting bulls through a winding 847m course.
Some participants say it's exhilarating.
"This run was very crazy, very fast," an American man told reporters.
"A brown bull had this guy by his pants, was dragging him down the street."
Of the three men the bulls caught, one was gored near his waist, another in his armpit.
People come from all over the world to test their bravery – or stupidity – as terrified bulls hurtle through the streets before arriving in the arena, where the animals are eventually killed in traditional bullfights.
The San Fermin festival, immortalised in Ernest Hemingway's 1926 novel The Sun Also Rises, dates back to medieval times and features religious processions, folk dancing, concerts and round-the-clock drinking.
Fifteen people have been killed in the bull runs since modern day records started in 1911.
The most recent death took place in 2009, when a bull gored a 27-year-old Spaniard in the neck, heart and lungs.
The United States, Australia, New Zealand and Britain accounted for the greatest number of foreigners. The vast majority of bull run participants are men.
3 News / NZN