The fiesta, an uproarious blend of hair-raising daily bull runs and all-night partying, was immortalised in Ernest Hemingway's 1926 novel "The Sun Also Rises."
Revellers wearing traditional white outfits trimmed with red neckerchiefs and cummerbunds gathered for today noon's launching of a firework rocket, which signals the beginning of the nine-day festival.
The regional government of Navarra said this year's festivities would be patrolled by 3,500 police to keep the adrenaline and alcohol-fuelled events as safe as possible.