The man, named by his lawyers as 30-year-old Rui Pinto, was detained in Hungary on Wednesday on a European arrest warrant filed by Portuguese police who want to extradite him. His lawyers said they would oppose this.
Speaking to Reuters, the lawyers said Pinto, whom they described as a "whistleblower", would remain under house arrest in Hungary until a decision on his extradition had been made.
Portuguese police said a national had been detained on Wednesday on a European arrest warrant. The individual was suspected of qualified extortion, violation of secrecy and illegally accessing information, the police said.
Asked by Reuters if the arrested man was Rui Pinto, Carlos Cabreiro, the Portuguese police's head of cyber crime, said he could not comment.
He also declined to comment on the lawyers' intention to oppose Pinto’s extradition.
The Football Leaks website includes emails, contracts and presentations relating to football clubs.
German magazine Der Spiegel, which shared access to the documents with Reuters and more than a dozen other media outlets
in cooperation with the European Investigative Collaborations (EIC) reporting project, said the material came from a source who identified himself as a Portuguese man named "John".
Reuters could not determine if Pinto was the source mentioned by Spiegel. Spiegel and EIC said in a statement that they do not comment on the identity of sources.
Reuters was also unable to determine what information the arrest warrant related to or whether it was linked to reporting by the consortium.
His lawyers, William Bourdon and Francisco Teixeira da Mota, said in a statement to the media that Pinto was not a criminal.
"(He) was outraged with practices in the sport (football), which he believes do not dignify the players and damage their image," they wrote.
Pinto, they said, became an "important European whistleblower for Football Leaks" and his revelations have "enabled numerous European judicial authorities to gain knowledge of criminal practices in the world of football".
The EIC started releasing information based on Football Leaks in late 2016, prompting the European Parliament to question representatives of football bodies.
(Reporting by Catarina Demony and Goncalo Almeida; Editing by Nick Tattersall and Jon Boyle)
