The bomb, which had been placed in a van parked outside a police barracks in the town of Durango, caused considerable damage to the building, a Civil Guard police spokesman said.
The two police officers were hit by flying glass.
A second vehicle, believed by police to have been used by the bombers to flee the attack, exploded in the town of Amorebieta about an hour later, said the spokesman.
There was no claim of responsibility for the attack, but the police spokesman said: “We have little doubt it was (ETA).’’
For weeks Spain’s Socialist government has warned of an imminent ETA attack after the arrests of members of the Basque guerrilla group and the capture of explosives. The guerrilla group said it was calling off a 15-month-old ceasefire, which it had effectively broken by bombing Madrid airport in December.