The sponsor, People for the Ethical Treatment of Animals, said more than 1,000 nude activists took part in the human run, now in its fifth year.
But that estimate seemed high, with the procession appearing to number just a few hundred, and very few of the runners actually naked. Some women ran topless or in panties and bras, and most of the men were in their underwear at least.
The run proceeded along part of the route that will see thrillseekers dash ahead of a pack of six fighting bulls and six meek steers starting Friday for eight days.
The protesters shouted slogans against the running of the bulls and against bullfighting in general as they made their way to the gate of Pamplona's bull ring, where the procession ended, the AP reports.
The bulls that take part in the running are corraled at the bull ring and killed in bullfights later in the day.
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