A bomb exploded Thursday in the offices of the governing Socialist party in the northern Basque city of Bilbao, injuring seven police officers, authorities said.

The blast occurred after a warning call to a regional traffic department was made in the name of the Basque separatist group ETA, a police spokesman said on condition of anonymity in keeping with police regulations.

ETA declared a cease-fire in March 2006, but ended it after it failed to win concessions in talks with Socialist Prime Minister Jose Luis Rodriguez Zapatero's government. It has since killed two police officers and one politician in shootings and has carried out several bombings.

The bomb exploded at 6 a.m. in the doorway of offices of the Socialist party in the district of La Pena.

The warning call came a half hour before the explosion and said where and when the blast would happen. But a police patrol in the area had already spotted a suspicious package outside the building and had begun evacuating residents and cordoning off the area of the package, the spokesman said.

The officers were still close to the building when the bomb went off. Seven were treated for minor injuries but none were hospitalized.

The explosion came a day after King Juan Carlos inaugurated Parliament in Madrid following March 9 elections.

Zapatero, who is beginning his second term in office, has called for full political party unity to help fight ETA.

ETA has killed more than 825 people since the late 1960s in its campaign for an independent Basque country in northern Spain and southwestern France.