The group claims the FBI may have violated European law and the Spanish Penal Code by shutting the file hosting service last weekend, which not only stopped alleged copyright infringement but prevented Megaupload users from accessing non-infringing personal data.
"By closing the service they have impeded the access to millions of archives of both private individuals and organisations, potentially causing huge personal, economic and image damages to a vast number of people," the group argues on the campaign's landing page.
It hopes to attract alleged victims in Spain that identify as a premium, lifetime or normal Megaupload user. However, the page will be used to collect responses from US residents too.
"This initiative is a starting point for legitimate internet users to help defend themselves from the legal abuses promoted by those wishing to aggressively lock away cultural materials for their own financial gain," it explains.
The Pirate Party of Catalonia also claims on its official website that the FBI's actions could conflict with European and UK privacy, data protection and computer misuse laws.
Besides endorsement from Pirate Party chapters across Europe, digital rights campaigner, the Electronic Frontiers Foundation, will support the campaign in the hope of discovering how the Megaupload closure had affected legitimate users in the US, an EFF spokesman told Ars Technica.
The manner and legal justification for the shutdown could set a precedent for users of similar file locker services.
Even without further action, several similar sites shut down or limited sharing functionality in the wake of Megaupload's closure, with the same consequences for their legitimate users.