The bombs detonated in Leon and Santillana del Mar in the north, Avila and Ciudad Real in central Spain, Alicante in the east, and Malaga in the south, the ministry said, adding it had no reports of injuries.
The news agency Europa Press said two people were slightly injured in Santillana del Mar and that the bomb went off in the center of the town, not the parking lot mentioned in the warning call.
The blasts followed two telephone warnings to the Basque newspaper Gara from callers claiming to represent ETA that said bombs had been placed in seven cities throughout the country. The last of the seven is Valladolid in the north.
Before the blasts, the Interior Ministry had said the seven sites targeted — mainly streets and plazas — had been evacuated and cordoned off.
after a similar call to Gara from a person claiming to speak for ETA. Damage was minor and two police officers were lightly injured. Gara often serves as a mouthpiece for ETA.
Another small bomb was defused Saturday in the southern city of Almeria.
Spanish security forces were on alert Monday, a public holiday marking the 26th anniversary of the passage of the Spanish constitution. The document laid the groundwork for Spain's system of granting broad autonomy to regions like the northern Basque country.
ETA wants outright independence for the region and has been blamed for more than 800 deaths in its decades-old campaign of violence.
In several of Monday's blasts, the targets were streets or plazas named after Spain itself, such as Avenida de Espana or Plaza de Espana.