At least a dozen young activists from various European countries and the United States said they wanted to draw attention to the suffering of the bulls during the animals' three-minute charge through the old city.
Their bodies painted with slogans like "let bulls die in peace," the activists stopped traffic and drew curious crowds with a call to ban bullfighting from the festival.
Hundreds of thousands of Spaniards and foreign tourists gather every year in Pamplona for the San Fermin Festival - the town's world-famous running of the bulls fiesta.
One woman sporting nothing but a pair of plastic horns said she felt uncomfortable stripping off, but that her suffering was nothing compared to that of the bulls, which are eventually slaughtered in the Pamplona bull ring.
The campaigners said they would make their nude protest an annual event.
Centuries old
San Fermin is a 400-year-old bullfight extravaganza that was immortalised in Ernest Hemingway's novel The Sun Also Rises.
Crowds jam the city's central square for the traditional firework display that starts a week of round-the-clock drinking, partying and a morning run with half a dozen fighting bulls.
Only a few hundred of the bravest gather in the old quarter each morning pursued by six stampeding bulls. The fighting bulls are killed by matadors each afternoon.
The bulls usually injure several people each day. Since the start of the last century, 13 runners have been gored fatally or trampled to death.
The running of the bulls dates back to 1591 when its purpose was to move the bulls into the arena.
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