The interior ministry on Monday said three people, including two police officers, were injured in the central city of Ciudad Real and two more in the northern Santillana del Mar.
Minor explosions also occurred in northern Leon, Valladolid and Avila and in southern Alicante and Malaga soon after 1.30pm (1230 GMT), the Spanish news agency Europa Press reported.
Anonymous callers claiming to speak for the underground Basque separatist group ETA had earlier told a newspaper to expect the explosions.
In Leon a bomb went off causing light damage in a cafeteria in a central square that had been evacuated after it was identified by the first caller, said local police.
Valladolid's central square, which was full of tourists celebrating a local holiday commemorating the adoption of Spain's constitution, was also evacuated and sealed off.
A low-intensity bomb went off in a cafeteria there that was closed, police said.
ETA warned authorities about
the explosions beforehand
One of the two bombs in Santillana del Mar went off in the parking lot of a zoo, the police said.
On Friday five bombs went off in filling stations around Madrid following a similar warning attributed to ETA, causing no casualties.
The warning was also telephoned to Gara, the usual vehicle for claims by ETA rebels.
Police on Sunday defused another device in the southern city of Almeria.