About 200 supporters of People for the Ethical Treatment of Animals came from 15 countries to march along the same cobblestone route that bulls and daredevil humans cover every July in a series of mad dashes from a holding pen to the bullring.
The protesters had planned to run totally nude but could not because they didn't get the necessary town hall permit, the Efe news agency reported.
PETA says the demonstration, its third annual "Running of the Nudes," is a gathering of "compassionate and fun-loving people from around the world" who travel to Pamplona to prove to the city "that it doesn't need to torture animals for tourism."
PETA is campaigning for a ban on both bullfighting and Pamplona's running of the bulls. The events - part of the centuries-old Mardi Gras-like Festival of St. Fermin - are traditions PETA argues should go the way of other formerly long-accepted practices such as slavery.
Isabel Gonzalez, head of an anti-bullfighting organization in Belgium, said bullfighting is cruel.
"The animals suffer, and nobody has the right to mistreat them just to stage a fiesta," she said.
She said the running of the bulls also is inhumane because runners pull the bull's tails, kick them and "the animals run in a state of panic."
This year's 204-hour-long festival begins Tuesday with a ceremony called the chupinazo, in which a small rocket is fired from a balcony overlooking the main square to signal the start of nine days of street parties in honor of the city's patron saint, San Fermin.
At least 14 people have died, and at least 200 have been injured, in the running of the bulls over the past 70 years.
This year's scheduled eight bull runs begin on Wednesday.