Authorities say there were 13 blasts at or near three train stations including the main Atocha terminal, in the southern part of the city. The blasts occurred early Thursday, during the morning rush hour.

There has been no claim of responsibility, but Spanish Interior Minister Angel Acebes told reporters he has no doubt the Basque separatist group ETA was responsible for the blasts that came just days ahead of Sunday's general elections. The leader of the outlawed Basque political party Batasuna, Arnold Otegi, has denied ETA carried out the attack, suggesting "Arab resistance" is responsible for the bombings. The Batasuna leader told Basque radio, ETA always calls in a warning before it attacks. Spanish authorities insist Batasuna is linked to ETA.

Earlier, some reports had sought to link the terrorism to Spain's strong cooperation with the United States in Iraq.

Meanwhile, all campaigning for Spain's election has been called off, and Prime Minister Jose Maria Aznar has declared three days of mourning for the victims.

If ETA is responsible for Thursday's attack it would be the deadliest ever carried out by the group. In 1987, an ETA bombing of a supermarket in Barcelona killed 21 people.

More than 800 people have been killed in ETA's 35-year armed campaign for an independent Basque state in northern Spain and southern France.