Spanish police are investigating after an 18-year-old woman reported that she was gang raped by a group of men in a park in Bilbao on 1 August.
Six men were arrested following the incident, but four were released after a judge said the woman had not provided a clear description of them.
The accused men are aged between 18 and 36 and are of North African origin, Spanish media report.
The woman was treated in hospital and discharged a few hours later.
None of the men have been charged.
Hundreds of protesters gathered in the city to condemn the release of four of the alleged group on Monday.
A court heard that the young woman had contacted one of the men on Instagram and had arranged to meet him in the Etxebarria park.
When she arrived at the park, she noticed that the man was not alone and attempted to leave. She was then prevented from doing so by the group of men, who allegedly raped her repeatedly. The woman then contacted the police.
This is the third case regarding sexual offences that has been reported in the Bilbao area in recent weeks, El Pais newspaper reported. On 26 June, three men were accused of abusing a 17-year-old girl in Iturribide.
Four days later, a 35-year-old man was arrested for assaulting an underage girl inside a van in Larraskitu.
In 2016, a gang rape that occurred at the San Fermin bull-running festival in the nearby city of Pamplona sparked outrage after a court acquitted five men of rape but convicted them of sexual abuse. The ruling in the La manada (the wolfpack) case was later overturned by Spain’s top court, and the conviction was upgraded to rape.