Two police officers were hurt when a bomb blamed on Basque separatist group ETA exploded in a city park in the northern Spanish city of Bilbao.
The bomb went off as the two officers went to remove a placard in the park, the Basque regional police said. State radio said the placard had slogans encouraging ETA to kill police.
No other details were immediately available.
ETA has killed more than 800 people in a 30-year campaign for a separate Basque state.
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The armed group last week claimed responsibility for the fatal shooting of a judge in northern Spain and a car bomb in Madrid which wounded more than 90 people.