At about 3am on Thursday, a woman was entering a doorway in the centre of the city in northern Spain when she was sexually assaulted, according to local newspaper Diario de Navarra.
The young woman was then treated at a nearby hospital while the police searched for the suspects. The investigation is still ongoing.
The town hall has called for citizens to gather at a central plaza for a protest organized by a feminist group at 9pm on Thursday evening to show solidarity against sexual crimes.
"We are not going to tolerate any sexual assaults at our fiestas," wrote local activist group Gora Iruñea on Facebook.
The city of Pamplona has been particularly focused on preventing sexual crimes at this year's traditional San Fermin bull-running festivals, organizing a special campaign to raise awareness and inform victims about their resources.
One particular concern is how the event is often depicted through souvenirs and other promotional materials, using images of women baring their breasts and being groped by sometimes dozens of men.
"The goal of our campaign is to take back public spaces for women, and to tell people that our bodies are not part of the party - our bodies are our own," campaign organizer Pilar Mayo told The Local on Wednesday.
"What happens at San Fermin is what people have inside their heads all year, but they think they have permission here to touch women, and women need to know they can say 'no'."
More police have been deployed this year, dark areas have been given more lighting and high-definition cameras have been posted to better identify perpetrators.