(Updates number of wounded, adds injury details)

MADRID, April 17 (Reuters) - A bomb exploded outside an office of the ruling Socialist Party in the Basque city of Bilbao on Thursday, wounding seven police officers who had cordoned off the area after a warning from ETA, police said.

The bomb, hidden in a bag, exploded at 6 a.m. (0400 GMT), half an hour after an anonymous caller told police where it had been placed and claimed responsibility for the Basque separatist group ETA, a police spokeswoman said.

Several policemen suffered ear injuries and one was hurt in a fall, she said.Others were lightly injured by broken glass from the blast which caused considerable damage to the Socialist party office, another spokesman said.

ETA has killed more than 800 people in four decades of armed struggle for independence for the Basque Country from Spain and France.

The guerrilla group's last fatal attack was on March 7 when, two days before a general election, it shot dead a former Socialist Party councillor in the Basque town of Mondragon.

It admitted responsibility for the killing on April 1. (Reporting by Jason Webb, Emma Pinedo, Joe Ortiz, Editing by Tim Pearce)

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