A small explosive device shattered glass at a ruling Socialist party office in Spain northern region of Asturias today, a day before a referendum on the EU constitution, officials said.
One media report attributed the blast to referendum opponents who consider the European charter "fascist".
A second homemade device packed with camping gas was found at the opposition Popular Party in Asturias, Interior Ministry under secretary Soledad Lopez told a news conference.
The Spanish news agency Europa Press, citing Interior Ministry officials, said a note found next to the exploded device criticised the European Constitution as fascist and a pact between the Socialists and the Popular Party.
The note was written in the Asturian dialect, Europa Press said. That would rule out the armed Basque separatist group ETA.
Some 106,000 police officers will be on alert during voting on tomorrow. Nearly 35 million voters are eligible to vote in the referendum to answer the question: "Do you approve the treaty establishing a Constitution for Europe?"
The vote will be watched by governments in other EU member states.