Basque militant group Eta says it planted four roadside bombs in northern Spain ahead of a general strike called by separatists in the Basque country.
The strike was called after the deaths of two Eta prisoners
Two of the bombs exploded and two were defused. No injuries were reported.
The banned Batasuna party called the strike in protest at the government's refusal to allow public tributes to two Eta members who died in prison.
Some roads and railway lines were blocked with burning barricades during the strike.
But Spanish Interior Minister Jose Antonio Alonso said the strike had not received much support in the Basque country and insisted the government would not be shaken by Eta's bombs.
"We must tell the terrorist organisation - if we must tell it anything at all - that the law-based state will not retreat a single millimetre in its fight against terrorism," he told Spanish Radio 1.
"Basque society has been totally calm and reasoned and has gone to work and to study; normal public life has not been called into question, there have been few incidents and backing for the strike has been negligible."
The Basque newspaper closed its website on the day of the strike.
A spokeswoman for the Basque regional government, Miren Azkarate, said 1% of government workers, 0.5% of health workers and around 4% of teaching staff had taken part in the strike. She mainly those close to Eta and Batasuna had backed the stoppage.
By planting the bombs, Eta had sabotaged its own day of action, she added.
"The way of bombs, violence and threats will not get us anywhere," she said. "It will not allow us to reach peace and normalisation."
Eta has been fighting for Basque independence since 1968. About 800 people have died in the fighting, although no-one has been killed by Eta for more than two years.
Eta has planted more than 12 bombs this year, despite reports it is planning to declare a ceasefire.