According to Interior Minister Jose Antonio Alonso on Tuesday, police arrested the latest suspect in the northern city of Pamplona.
Just 24 hours earlier, security forces seized seven suspects in raids across the country. "They were talking about an attack on the High Court or another judicial body but police are not ruling out other possible targets," Alonso said.
The High Court is handling the cases of the suspected Madrid train bombers - an attack that resulted in 191 deaths.
But investigators have not found any evidence of direct contacts between the latest detainees and the train-bombing suspects, the daily newspaper ABC said.
The minister also said police had not found weapons or explosives in their raids, and that the operation was ongoing.
The seven suspects arrested on Monday, including four Algerians and one Moroccan, were detained in the southern region of Andalusia, the Mediterranean city of Valencia and Madrid, according to an interior ministry statement.
They were residents in Spain and most of them had served time in prison. They were in contact with other individuals in Europe, the United States and Australia, the statement said.
One of the men arrested was an informer for Spain's intelligence service, El Pais newspaper reported.
Police started investigating the suspects during the summer, and have wiretaps in which the men discuss the need to organise a major attack with explosives, Spanish news agency Europa Press said, citing police sources.
None of the conversations were with people who could have provided explosives, the agency said.