Spanish authorities say a pre-dawn bomb blast Sunday in the Basque town of Elgoibar damaged the local offices of Spain's ruling Socialist party.
A warning call was made before the explosion in the name of the Basque separatist group ETA.
No one was hurt as police received the telephone warning in time to clear the area.
The attack is similar to Thursday's bombing of the Socialist office in the city of Bilbao. Police received a warning call from ETA, but seven officers were wounded in that blast.
ETA is an armed separatist group that has been fighting for 40 years to establish an independent Basque state in northern Spain and southwestern France. The group is blamed for the deaths of more than 800 people.