Eta has been blamed for nine attacks so far this year, including this week's blasts, although there has been no official confirmation the separatist group carried them out.
Hunt for perpetrators
On Wednesday, a car bomb outside a police barracks in the city of Burgos, in Spain's north, injured about 46 people, blowing off the face of the building.
The following day a blast killed two police officers in Palmanova on the island of Mallorca, and police defused a second bomb underneath a nearby civil guard vehicle.
Officers Carlos Saenz de Tejada Garcia, 28, and Diego Salva Lesaun, 27, died almost instantly when a bomb exploded under their patrol car.
A memorial was held for them in Palma de Mallorca on Friday, attended by the leaders of Spain's main political parties, alongside members of the royal family.
The government has blamed the deaths on Eta, as a search for those behind the Mallorca blast continued.
The interior ministry has issued photographs of six suspected Eta members and called on the public to track them down.
While both blasts attacked civil guards, a common Eta target, there were no customary warning calls before either of them.
'Campaign of bombing'
But Jonah Hull, Al Jazeera's correspondent in San Sebastian in the Basque region, said: "No-one is in any doubt, here in Spain, that this was the work of Eta.
"This has become in recent years something of a common practice for Eta to launch a campaign of bombing during the summer, at the height of the tourist season, and certainly not confined to the Basque region.
"The fear nationwide now is that this is the beginning of something that might be far more sustained."
Spain has vowed to crush the separatist group since Eta ended what it had said was a permanent ceasefire with a 2006 bombing that destroyed a Madrid airport parking garage and killed two people.
The group was founded on July 31, 1959, and began its violent campaign in 1968 for a separate state to be carved out of northern Spain and southwestern France.