"The victims, mainly vulnerable young Nigerian women, were recruited in Nigeria and trafficked to Spain via Italy," Europe's Hague-based policing agency said.
"Once in Spain they were forced into prostitution, mostly in the cities of Benidorm and Malaga, but also Madrid, Barcelona, Soria and Gandia (Valencia)," Europol said in a statement.
Spanish police arrested the gang members in several Spanish cities, but its leader was taken into custody in Helsinki, where "she was arrested by the police, thanks to effective international cooperation."
The probe was started when authorities were alerted that two Nigerian women who were seeking asylum could be victims of human trafficking, Europol said.
"Investigations revealed the existence of an organised crime group operating in Spain that had a big infrastructure in Nigeria, as well as links in Niger, Libya and Italy."
The women were smuggled to Spain where they were told to seek international protection and asylum "so that they could work for the criminal organisation without problems in the event of being identified by the police," Europol said.
The group provided the victims with fraudulent documents to request asylum, it added.
Spanish police earlier this month busted another network that was forcing transsexuals into prostitution, after luring most of them to Spain from Venezuela.