Tens of thousands of Spaniards took to the streets of Pamplona on Saturday to protest against the acquittal of five men accused of gang raping an 18-year-old woman at the city's bull-running festival.

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.

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.