Spanish officials have expressed surprise a car bomb which destroyed a police barracks did not kill dozens of people, including children.
The device went off at about 0400 local time (0200 GMT), and tore the front off the high-rise Civil Guard building in the northern city of Burgos.
Some 46 people, many of them the wives and children of Civil Guards, were injured, mainly with flying glass.
Spain's interior minister blamed the Basque separatists Eta.
He said the group had been "undoubtedly trying to kill" people.
Alfredo Perez Rubalcaba, speaking from the scene, said no telephone warning had been issued.
He said the attack was not only aimed at the guards, "but also their families, which makes this particularly despicable".
Steve Kingstone, Madrid correspondent
This attack is a sobering reminder of Eta's potency, to politicians and commentators who had previously portrayed the group as weakened following a series of high-profile arrests.
Those arrests certainly dented Eta's leadership structure, but this attack demonstrates that other militants are willing to step into the shoes of those now in jail. And the quantity of explosives used - almost 300kg, according to Spanish reports - makes clear that the group still has the potential to inflict violence on a large scale.
The catalyst for this bombing was the exclusion of separatist parties from regional elections in March. Eta branded the vote "undemocratic," and described members of the resulting Socialist administration as a "priority target." The Socialists ended almost three decades of rule by the Basque Nationalist Party, and have promised renewed vigour in tackling the militants.
"Last night 120 people were sleeping [in the barracks] and 41 children could have been killed," he said.
Eta has fought for an independent state in northern Spain and south-western France since 1968, in a campaign that has cost more than 820 lives.
Although there was no telephone warning, unlike in previous Eta attacks, correspondents pointed out that the target and means of attack were consistent with Eta's track record.
Local politicians said the group had obviously hoped to inflict serious damage and multiple casualties.
"All the windows are broken. It's very damaged so it's almost a miracle no-one was hurt more seriously," an emergency services spokesperson said.
The bomb made a crater 7m (20ft) deep and spread wreckage over a wide area.
"The car used to cause the explosion has been displaced some 70m (230ft), so that gives you an idea of the power of the blast," said regional ministry representative Miguel Alejo.
The location of the attack may cause concern for the authorities, says the BBC's Steve Kingstone in Madrid.
Burgos is one of Spain's great historic cities, popular with tourists, especially in summer, he says.
The power of the explosion shows that, despite several high profile arrests in recent months, Eta still has ample means to continue its campaign of violence, our correspondent adds.
Eta ended a ceasefire two years ago.
Last month it was blamed for a car bomb attack in the northern city of Bilbao in which a senior police officer was killed.
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