Police have arrested nine people suspected of leading a group considered part of the violent Basque separatist organisation Eta, Spain's Interior Ministry said.
Tuesday's pre-dawn raids marked the first Eta-related arrests since the group announced a new ceasefire on September 5.
The government responded to the announcement with scepticism and said it would not ease up on Eta or those close to it.
A ministry statement said the new arrests dismantled the core leadership of a Basque civic organisation called Ekin.
Spain's National Court ruled in 2007 that Ekin is in effect part of Eta, and declared Ekin to be illegal.
A ministry official said the detainees are accused of offences including trying to resurrect Ekin.