Falciani’s passport was seized and he has to report to the court each week while his extradition request is considered, a source told news agency AFP.
In 2008, the French-Italian national passed the stolen HSBC filed to French authorities.
The documents indicated that HSBC helped more than 120,000 clients hide €180.6bn (£161.3bn, $199.8bn) between November 2006 and March 2007.
Falciani moved to Spain in 2012, with a local court rejecting a 2013 extradition request from Switzerland on the grounds that the crime of which he was accused – namely breaking banking secrecy laws – was not subject to prosecution in Spain.
In late 2015, he was sentenced in absentia to five years in prison by a Swiss court for industrial espionage, data theft and violating commercial and banking secrecy.
In an ironic twist, he was apprehended on Wednesday as he was about to speak in a debate on the need to protect whistleblowers.
“We were going to debate the fact that Spain is one of the European countries where fewer measures have been implemented to protect whistleblowers,” the president of Spain’s tax inspectors union Gestha, Carlos Cruzado, told AFP.
According to the PA, Falciani’s arrest comes at the same time Spain is seeking the extradition of Marta Rovira from Switzerland.
A prominent Catalan separatist politician, Rovira is seen by many as a key player in the Spanish region’s bid for independence.
Falciania’s lawyer in Switzerland, Marc Henzelin, said one “hypothesis” is that the arrest could be connected to a “sort of deal” to swap Rovira and Falciana, although this could not be verified.
“I’m not able to verify that, but frankly if it were the case, I would consider that rather odious,” he said to the PA.