Five people suspected of being part of a logistics team for Basque separatists Eta are being held in northern Spain.
The five were arrested in dawn raids in northern Spain
The four men and a woman are suspected of helping Eta members cross borders into and out of Spain, police say.
The house searches and arrests took place in Arrigorriaga, Sopelana, Berango and La Baneza early Wednesday.
On Monday, Eta released a statement saying it would continue its violent campaign for self-determination in the north of Spain.
The video message by three masked and armed militants said they were fighting for the Basques' right to decide their own political future.
Spanish Interior Minister Jose Antonio Alonso told reporters in the north-western city of Santiago de Compostela that the group arrested on Wednesday "were part of a structure of Eta that allows transferring people between Spain and France".
"Police have seized diaries, computers, Eta membership documents... material that is being analysed as the operation is not over," he added.
Eta has largely kept a low profile since the Madrid train bombings by Islamist militants in March which killed 191 people.
The Spanish authorities initially blamed the group for those attacks, later saying they had been organised by Islamic extremists.
Eta has claimed, or been blamed for, more than 800 deaths since launching a campaign in the late 1960s for an independent Basque homeland straddling northern Spain and south-west France.
But the group is thought to have been weakened following a series of high-profile arrests.
These were largely due to improved co-operation between the Spanish and French police forces following the 11 September 2001 attacks on New York and Washington.