MADRID — The northern Spanish city of Pamplona has become famous worldwide for its high-risk festival in which revelers run through narrow streets in the early morning with charging bulls on their way to the ring.

But the annual festivities and street drama have taken a darker turn this year after 15 people were arrested over the past week in separate cases of sexual assault, including gang rape.

The arrests have set off mass protests, including on Monday evening, when thousands of people interrupted the festivities to denounce the attacks. Many held up signs showing an open hand painted in red with the slogan “No is No.”

The episodes come at a time of heightened awareness and mounting anger over sexual assaults in Europe in a year that began with scores of attacks during New Year’s Eve celebrations in Cologne, Germany. Migrants were largely blamed for the attacks. Germany recently passed a strict new law that would make it easier to prosecute suspects of sexual violence.