Demonstrators have filled streets across the country since the court ruling on Thursday, leading Spain's conservative government to say it will consider changing rape laws.
The men were acquitted of sexual assault, which includes rape, and sentenced to nine years for the lesser offence of sexual abuse.
In Pamplona itself, police said that "between 32,000 and 35,000 people" took part in a demonstration on Saturday, rallying under the slogan "it's not sexual abuse, it's rape".
Thousands of women marched together with their hands raised at the protest, which police said passed off peacefully.
Ana Botin, the influential head of Santander, one of Spain's biggest banks, tweeted that the ruling was "a step back for women's security" while former judge Manuela Carmena, now Madrid mayor, said it "does not meet women's demand for justice."
The men, aged 27 to 29, had been accused of raping the woman at the entrance to an apartment building in Pamplona on July 7, 2016, at the start of the week-long San Fermin festival, which draws tens of thousands of visitors.
The five, all from the southern city of Seville, filmed the incident with their smartphones and then bragged about it on a WhatsApp messaging group where they referred to themselves as "La Manada", or "The Pack" in English.