Daredevils, many in traditional white clothing with red kerchiefs around their necks, tripped over each other or fell in the mad dash through the narrow, cobbled streets of the northern Spanish city of Pamplona.
No one was gored but a 23-year-old Briton from Nottingham fractured several ribs and injured his chest during the run, regional health authorities said. He was in a serious condition in hospital.
A 23-year-old Japanese man from Okayama who suffered bruises to his knee and elbow and a 36-year-old Pamplona native who bruised his back were also taken to hospital. Their injuries were deemed to be light.
The fighting bulls from the Dolores Aguirre ranch took just two minutes and 22 seconds to cover the 850-metre (2,800-foot) course from a holding pen to Pamplona bull ring where the beasts will face matadors in an evening bullfight.
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Several runners crashed against a wall in a curve along the route and were trampled by the bulls.
"I managed to run alongside the bull and then fell over. Some bloke pulled me under the wooden fence and out of its way," said John Ross, 41, who came from Manchester for the festival and whose knees were bloodied from the fall.
"The first 15 seconds were good. Then I tumbled and people piled into me. I am the guy who people are going to blame for causing them to fall," he said at a bar near the bull ring after the run.
One young man dressed in white fell to the ground at the entrance to the bull ring and clasped his head in his hands as the bulls raced by.
"It was war. People pushed and grabbed onto you," said Javier Alvarez, a 23-year-old from Pamplona who wore a white Real Madrid jersey with the name of German midfielder Mesut Ozil on the back.
Another four Spaniards were taken to hospital for various injuries, two of them in serious condition.