Seven small bombs have gone off in urban areas across Spain, following warnings from Eta Basque separatists.
Monday's attacks across Spain follow blasts in Madrid on Friday
Blasts were reported in Leon, Avila, Santillana del Mar, Ciudad Real, Malaga Valladolid and Alicante.
The Basque newspaper Gara said it had received two calls from people claiming to be from Eta, warning of seven bombs on Monday - a public holiday in Spain.
Police had started evacuating areas listed by the callers, but some people were slightly injured.
The callers gave precise descriptions of where the devices would be detonated. The interior ministry said that meant the areas listed - which included cafes, bars and town squares - were evacuated and cordoned off.
Police say all the explosions were extremely small and only minimal damage was reported.
A woman from Tenerife said her family were among those hurt as the bomb went off in the northern Spanish coastal town of Santillana del Mar.
"We had no idea a thing like this could happen. Initially we couldn't make out anything at all, and we only realised what had happened just afterwards," she told Spanish National Radio.
"All of us were affected - a bit of shrapnel in the legs, a small cut in my mother's ear, and we all had problems hearing."
Officials in Santillana del Mar said 10 people in all suffered hearing problems as a result of the blast at a zoo car park in the town, according to Reuters. There are no reports of injuries elsewhere.
Monday's co-ordinated attacks followed five simultaneous explosions at petrol stations along the main motorways leading out of Madrid on Friday.
The blasts came during a bank holiday to mark the 1978 constitution, which heralded a return to democracy after four decades of military dictatorship.
Eta, in its fight for Basque independence in the north of Spain, opposes that constitution and police had warned of possible attacks.
Eta often calls the Gara newspaper ahead of attacks, as it appears to have done on Friday and Monday - preventing serious injuries both times.
Former Eta member Teo Uriarte, who now leads a peace association in the Basque region, said Eta wanted to "sow fear".
"It wants to draw attention, but cannot afford to kill people. That's why they set off bombs the size of a bar of chocolate," he told the Associated Press.
The BBC's Katya Adler in Madrid says the attacks look like a message from Eta to show that it is still alive and dangerous despite the recent arrests of key members and the seizure of arms caches.
However, she says it is widely accepted in Spain that the group is at the weakest point in its history, which has resulted in more than 800 deaths over the last 30 years.
Following the attacks on Monday, Prime Minister Jose Luis Rodriguez Zapatero said Eta had only one choice: to "stop the violence and hand in its weapons".
The head of the opposition conservative Popular Party (PP) Mariano Rajoy said he stood by the Socialist government's efforts to neutralise Eta.
Leftist Basque party Ezker Batua-Berdeak also condemned the attacks and appealed to Batasuna - the political wing of Eta - to speak out, which it failed to do after Friday's bombings.
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