Whoever set off the bomb in Durango, about 40 kilometers (25 miles) south of Bilbao, fled in a car and then detonated it in a vacant lot in the nearby town of Amorebieta-Etxano, according to the Spanish Interior Ministry office in the Basque capital, Vitoria-Gasteiz.
The blast caused serious damage to the Civil Guard station and residential barracks in Durango, shattering windows and damaging police cars parked outside, authorities said. Several nearby apartment buildings were also damaged.
The attack happened shortly after 3:15 a.m. (0115 GMT). There had been no warning, the Interior Ministry said.
Police believe the bomb contained some 80 kilograms (175 pounds) of explosives and was packed with shrapnel, the Spanish national news agency Efe said. It was not immediately known how many people were in the station at the time of the attack.
The injured officers were treated for cuts from flying glass, but were released from a hospital in a nearby town several hours later. ETA called the cease-fire in March 2006, but grew frustrated with a lack of government comitment in ensuing peace talks .