(Adds explosives find)

MADRID, March 28 (Reuters) - Spanish police arrested eight suspected members of armed separatist group ETA on Wednesday, Interior Minister Alfredo Perez Rubalcaba said.

The Socialist government has been criticised for going soft on ETA and Batasuna, a political party banned for its links to the separatists.

"The civil guard has been carrying out a major operation against the infrastructure of the terrorist group ETA. So far eight people have been arrested," Rubalcaba said on state radio.

He said police were raiding more houses in the northern region of the Basque Country and neighbouring Navarre.

Radio reported one of those arrested was a known ETA member who had been on the run since 2005 and who has a conviction for assaulting a policeman.

Police later found 30kg of explosives in one of the raids, along with detonators and limpet devices, Spanish media quoted security sources as saying.

The arrests came a day after a new Basque party called Abertzale Sozialisten Batasuna registered to run in regional elections called for May.

On Tuesday, the Spanish government said it would investigate whether ASB was a vehicle for Batasuna, outlawed for not renouncing violence. The opposition Popular Party said it would fight to prevent Batasuna from running in the elections.

Spain's government is working to end ETA's bloody four-decade campaign but froze contacts with the separatists in December after a bomb at Madrid airport killed two people.

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