A bomb planted by suspected activists of the militant Basque separatist group ETA exploded overnight in the Basque town of Lazkao, causing no injuries, regional officials said Monday, dpa reported.
The blast did, however, cause serious damage to the local headquarters of Spanish Prime Minister Jose Luis Rodriguez Zapatero's Socialist Party, which is believed to have a good chance of winning Sunday's regional elections.
An anonymous caller speaking in the name of ETA warned a road rescue service about the bomb 1.5 hours before it went off.
Police had previously spotted a suspicious bag near the Socialist Party office and cordoned off the area. Initial estimates put the weight of the explosives at about five kilograms.
The vote to elect a 75-member regional parliament will be the first one without the participation of radical separatists close to ETA, after the Supreme Court barred two such formations from contesting the poll.
The court decision was followed by a car bombing in Madrid that caused no injuries on February 9.
The Basque elections are believed to be the first ones in nearly three decades that could remove the moderate Basque Nationalist Party (PNV) from power.
The PNV now faces a serious challenge from a mainstream Spanish party after Zapatero's socialists increased their support in the region.
ETA, which regards the PNV as too moderate in promoting the Basque nationalist cause, does not want either party to win.
Radical separatists have staged protests against the judicial bans on their electoral lists, interrupted rallies or booed candidates of the PNV and socialists, and staged minor attacks against PNV offices.
ETA, which is regarded as a terrorist organization by the European Union and the United States, has killed more than 800 people since 1968.