MADRID, Spain (CNN) -- A powerful bomb preceded by a warning call exploded in Spain's northern city of Bilbao, the largest Basque city, early Thursday, injuring numerous police officers. Officials blamed the attack on the Basque separatist group ETA.
"It was an authentic bomb," the Basque regional government interior minister, Javier Balza told the Spanish radio network SER.
A "high number" of Basque regional police officers were injured in the blast outside a ruling Socialist Party locale, Balza said. News reports said seven officers were slightly hurt. The locale was also damaged.
A Basque police patrol located the bomb even before a warning call in the name of ETA, Balza said. Officers rushed in to evacuate people from their homes, including "senior citizens, and little girls and boys," he said.
The bomb went off around 6 am. There was no immediate claim of responsibility. ETA typically makes a statement days or weeks after an attack.
The bomb blast comes a day after Spain's parliament opened a new session. Watch King Juan Carlos and PM Juan Luis Rodriguez Zapatero usher in the new session »
The attack was also the second important instance against the Socialist Party in five weeks. On March 7, two days before Socialist Prime Minister Jose Luis Rodriguez Zapatero won re-election in nationwide balloting, ETA was blamed for the fatal shooting of a former town councilman from the Socialist Party in a northern Basque town.
ETA is blamed for more than 800 deaths in its long fight for Basque independence.
The group declared a unilateral cease-fire in March 2006, raising hopes for an end to the violence. However, ETA's bomb at Madrid's airport in December 2006 killed two men and destroyed a parking garage -- and ended the peace process.
ETA officially terminated the cease-fire in June 2007, and since then the government has blamed it for three killings: two undercover Spanish Civil Guards who were shot dead in France while on an anti-terrorist operation against ETA, and the former councilman killed on March 7. E-mail to a friend