More than 1,000 people - mostly young men dressed in the region's traditional white shirt, trousers, red kerchief and sash - ran through the streets with six fighting bulls.
One of the animals gored Justin Hammerback, 32, of Chicago, leaving a 6-inch wound to his buttocks, officials said. Hammerback was hospitalized but his life was not in danger.
A Canadian man, Robert Stodola, was treated and released after being gored in his right elbow, and an Australian man was hospitalized with a gash in his thigh. Three Spaniards also were hospitalized with injuries.
Many others were trampled by the stampeding bulls or fellow runners during the race along the 825-yard route from the corral to the city's bullring.
One bull became separated from the pack and knocked a runner to the ground ripping off his trouser leg. The bull finished the run with the remains of the trousers still stuck to his left horn.
The early-morning runs, or encierros as they are known in Spanish, are the highlight of the eight-day San Fermin Fiesta. Thousands of people test their bravery and agility by running alongside six fighting bulls and a handful of accompanying steers as they are herded from the corral to the bullring.
The bulls are invariably killed by professional matadors in bullfights later each day during the fiesta.
The centuries-old festival, which features nonstop street partying, became internationally famous with Ernest Hemingway's 1926 novel The Sun Also Rises. Tens of thousands of foreigners flock to the northern Spanish city each year for the event.
Overcrowding, and the fact that many of the runners spend the night drinking, has made the runs even more dangerous. About 13 runners have been killed and more than 200 injured since records began being kept in 1924. The last fatality was an American in 1995, the first death since 1980.
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