According to a police spokeswoman, the device exploded at the back of the ruling Socialist party office.
The blast caused damage to the office and smashed windows in nearby houses and cars.
The device was made of about 5kg of explosives, police said.
Eta set off a similar sized bomb on December 16 outside a courthouse in the Basque town of Sestao.
Last week, a Spanish court convicted 47 members of various Basque separatist groups over their links to Eta.
The organisation said it shot two Spanish police officers in France earlier this month.
The Socialist party and the Popular party, the opposition, have been divided in recent years over measures to combat Eta, whose members have killed more than 800 people since the late 1960s in its campaign for an independent Basque homeland in a region in northern Spain and southwest France.