Basque separatists today exploded their second bomb in four days, after a two-month lull which had fuelled hopes of a truce.
The blast at a disused electricity substation near Zaragoza in northeast Spain caused slight damage but no injuries.
The Basque highway agency received a warning phone call from outlawed separatist group ETA, giving police time to cordon off the area before the bomb went off at about 8am.
Hours earlier ETA had issued a statement in which the outlawed organisation said it saw a chance of reaching a "democratic situation" which recognised the rights of the Basque country.
On Saturday night, a car bomb exploded at an industrial park in central Spain, causing no injuries. Two Basque newspapers had received calls in the name of ETA warning of the bomb.
ETA, classed as a terrorist group by Spain, the EU and the US, has killed nearly 850 people since 1968 in a bombing and shooting campaign for an independent Basque state in northern Spain and southwestern France.