Spanish Red Cross spokesman Jose Aldaba said no one was gored but one runner was taken to a hospital with a suspected bone fracture.
Oscar Gorria, deputy director of surgical services for the region of Navarra, said one man sustained an arm injury while another had cuts and bruises to a leg.
Municipal workers had applied an anti-slip coating on the cobblestones of the narrow streets in a bid to lessen injuries after rain had fallen overnight.
Weekend runs traditionally attract the most thrill-seekers keen on running the 930-yard course from a holding pen to Pamplona's bull ring.
Juan Arnal, a city official responsible for street safety, said the large turnout despite heavy rain spurred the decision to spray the cobblestones.
Even so, some of the massive bulls lost their footing. Television footage showed one lucky escape as a man had his T-shirt ripped off by a horn as one of the beasts lifted itself from a tumble.
The festival, which blends early-morning adrenaline with all-night parties where wine flows freely, attracts tens of thousands of foreign tourists each year. Fifteen people have died from gorings since record-keeping began in 1924.
Four Spaniards and an American have been gored so far during this year's festivities. One of the Spaniards was seriously injured.
The nine-day festival was immortalized in Ernest Hemingway's 1926 novel “The Sun Also Rises.”
The largest turnout this year is expected Sunday when the bulls come from the renowned Miura breeding ranch. They are some of Spain's largest and most fearsome fighting animals, weighing up to 1,530 pounds.
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