The mercenaries are fighting in far-right paramilitary militias, including in the Azov battalion alongside Kiev forces and other radical volunteers, the detained Spaniards said.
"We have clues that there are Spaniards in the neo-Nazi Azov battalion," Sergio V., one of the detainees, said, who was fighting in eastern Ukraine between September and December 2014.
Ramiro Gomez, head of the Madrid Committee in Support of Anti-Fascist Ukraine points out that Spanish justice has double standards, only accusing Spaniards who support the anti-fascists.
"We know there are mercenaries, who used to be professional servicemen in Spain, they are fighting together with fascist groups and get paid. But nobody is going to prosecute them," Gomez said.
Gomez also said there were branches of Ukraine's largest commercial bank PrivatBank in Spain, saying that payments to the neo-Nazi volunteers proceed through this bank.
However, Agustin Rivas, a representative of the Madrid Committee in Support of Anti-Fascist Ukraine, confirmed to Sputnik on Friday that all eight Spanish citizens fought alongside independence supporters.
Rivas added that the accusations against the detained had "nothing to do with reality," stressing that what happened was "a manifestation of double standards" since no Spanish citizens who fought alongside Kiev were arrested.
All the detainees were released following interrogation.