The crackdown is another setback for Socialist Prime Minister Pedro Sánchez's party, which has been hit hard by a series of corruption scandals.
The police operation comes as a result of a court order to find material linked to an investigation into corruption allegations against a former party member, the Civil Guard said.
Another court announced last week that former Prime Minister José Luis Rodríguez Zapatero is being investigated for alleged influence peddling and other possible crimes. He has denied all charges.
