The blast occurred after an anonymous caller claiming to represent the Basque separatist group ETA phoned authorities about the presence of the explosive, El Pais newspaper reported online.
No one was hurt in the attack.
For more than three decades, ETA has been fighting for an autonomous homeland along the Spanish-French border. More than 850 people have been killed in ETA-related violence during that time.
ETA said last month it was calling off the year-old cease-fire with the government because of failed peace talks.
Spanish Prime Minister Jose Luis Rodriguez Zapatero conceded that the beginning of 2007 the peace process with ETA was not proceeding as he had hoped.