Suspected militants from the Basque separatist group ETA bombed a television mast near Bilbao on Saturday, causing material damage but no injuries, Spanish police said.
Police had received information about the explosive device from an anonymous phone caller speaking on behalf of ETA approximately an hour before the explosion.
The call was an attempt to lure the police into a trap as the bomb, which was approximately four kilos, was detonated as bomb disposal experts approached with their equipment.
Spain on Thursday raised its terrorism alert to the maximum level ahead of the March 9 elections, the interior ministry said.
Security was being stepped up in places linked to the elections, such as party headquarters and rally venues, as well as shopping centres, transport hubs and other crowded places.
Interior Minister Alfredo Perez Rubalcaba said he expected ETA to attempt an attack, though he described the group as very weak and almost defeated.