The bulls run alongside six steers, or castrated bulls, chasing people dressed in white with red neckerchiefs through the streets into the town's bullring. Later in the day they will be killed in a bullfight.
Runners are often injured when they trip up or barge into each other in the crammed streets of Pamplona.
The San Fermin festival dates back to the Middle Ages and has religious origins. Participants still chant to an image of the saint and ask for his blessing before the run, which starts at 8 o'clock.
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