A device has exploded outside the regional headquarters of Spain's main opposition party in the city of Valencia.
No injuries were reported and police have cordoned off the area.
Spain is due to hold municipal elections across the country on 27 May.
Socialist Prime Minister Jose Luis Rodriguez Zapatero held a rally in the city this morning.
Spanish news agency Europa Press said the device that exploded consisted of a vacuum flask attached to a bottle with inflammable liquid and cited sources saying some sort of anti-globalisation group could be behind the attack.
Attacks on political party offices are relatively frequent in the Basque Country region in northern Spain.
Youth groups linked to armed group ETA carry out sporadic attacks using Molotov cocktails or homemade explosives.
ETA has killed 800 people in 40 years of bombings and shootings to back its demands for an independent Basque state.
The government scrapped peace talks with ETA in December last year when the group bombed Madrid airport, killing two people in its first fatal attack since May 2003.
