The five who called themselves "the pack" were sentenced on Thursday to nine years in jail for sexual abuse, which sparked protests in Barcelona, Madrid and other cities including Seville, the men's hometown.
On Friday, demonstrators again gathered outside the Pamplona court which handed down the verdict, shouting "it's not sexual abuse, it's rape" and holding banners reading "palace of injustice".
Further demonstrations are planned in the city on Saturday, the Pamplona Woman's Movement said.
The men, aged 27 to 29, had been accused of raping the woman, then aged 18, 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 gang left her half-naked after stealing her mobile phone.
The five 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,.
Judges acquitted them of sexual assault, which includes rape, because they ruled there was no "violence or intimidation" in the case.
The judges also ruled that the victim had neither consented nor expressed any opposition.
The angry backlash against the verdict follows the wave of feminist protest movements around the world sparked by the #Metoo campaign against sexual harassment and assault.
Possible law reform
The Spanish government on Friday announced it would consider the possibility of reforming the law.
Several left-wing politicians questioned the verdict while the regional government of Navarre province said it would launch an appeal because it "shares the discontent of the citizens".
The prosecutor's office said it will also challenge the judgement.
Prosecutors had argued during the trial last autumn that there was "serious intimidation and it prevented resistance or flight".
They had asked for each of the accused to be jailed for 22 years and 10 months.
One of the accused is a Guardia Civil policeman -- currently suspended -- and another is a former soldier. Several are "ultras" who support FC Sevilla.
The five men who have been in pre-trial custody since they were arrested were also ordered to pay the victim 50,000 euros ($61,000) in compensation.
Defence lawyers had argued the victim agreed to the sexual encounter and would only acknowledge the gang had stolen the mobile of the girl, who was left half naked in the building entrance.
Lawyers for the woman and attorneys for four of the men said they would appeal.
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