Amid a wave of grief and revulsion, Spaniards placed candles and flowers at the Santa Eugenia station in southeastern Madrid on Thursday where one of the blasts occurred.
Vigils of sympathy and protest were held in Madrid, in Bilbao and Vitoria in the Basque region and Barcelona, where thousands came onto their balconies to bang pots and pans.
In central Madrid, more than 1,000 people gathered and shouted slogans against ETA, according to news reports.
In Barcelona, scores of protesters held up signs saying "No ETA" while in Seville, 2,000 protesters waved banners saying "No more death" and "Murderers, that's enough".
The Spanish government said it believed ETA was most likely to blame for the simultaneous bombings of four trains at Madrid stations, three days before a general election.
The leader of ETA's banned political wing Batasuna, Arnaldo Otegi, said of Aznar: "He is lying and he knows who did it.... He wants to maintain this hypothesis until after Sunday's election."
Batasuna is not permitted to participate in the upcoming elections
Spanish Foreign Minister Ana Palacio told French radio on Friday that "everything appears to indicate" that the Basque group ETA was behind the blasts.
But Interior Minister Angel Acebes said police were not ruling out any lines of investigation after finding a van containing seven detonators and a tape in Arabic at a town near Madrid where the bombs may have been placed on the trains.
"The hardest thing was hearing mobile phones ringing in the pockets of the bodies. They (paramedics) couldn't get that out of their heads."
Madrid regional government
Europe's worst attack
Apart from those killed, some 1,421 people were injured in Europe's bloodiest attack for more than 15 years.
But blame was clouded late on Thursday when a letter purporting to come from al-Qaida claimed responsibility.
"We have succeeded in infiltrating the heart of crusader Europe and struck one of the bases of the crusader alliance," said the letter sent to London-based newspaper al-Quds al-Arabi.
But the letter has not been authenticated.
An Interior Ministry source said officials were looking into the claim but ETA remained the first line of investigation.
Investigators say there were 10 blasts. The bombs, in rucksacks, each contained around 10 kg of explosives.
El Mundo newspaper said Madrid had suffered "the worst terrorist attack in Spanish history", calling it "Our September 11".
Spanish Prime Minister Jose Maria Aznar called for mass demonstrations in cities across Spain on Friday evening in rejection of the attacks.
Witnesses described scenes of horror and carnage at the three Madrid stations hit by the blasts.
Demonstrated protested against
the Basque separatist group
"The train was cut open like a can of tuna," ambulance driver Enrique Sanchez told reporters at the huge Atocha station in central Madrid. "There were all kinds of facial wounds, amputations, broken bones."
Hundreds of people whose loved ones took the pre-dawn train to Madrid rushed to identify bodies and scour lists on hospital notice boards. A makeshift morgue was set up at a convention centre.
Esperanza Aguirre, head of the Madrid regional government, told state radio forensic scientists had identified about 50 bodies so far.
She said the chief forensic scientist had told her that "the hardest thing was hearing mobile phones ringing in the pockets of the bodies. They couldn't get that out of their heads."
If ETA is responsible it would be by far the bloodiest attack carried out by the group, which has killed around 850 people since 1968 in its fight for a separate Basque homeland in northwest Spain and southwest France. ETA has been branded a terrorist group by the United States and the European Union.
Friday should have been the culmination of Spain's general election campaign which has focused on Spain's unity, threatened by calls from Basque and Catalan nationalists for more autonomy.
Instead, the major parties suspended their campaigns.
Aznar vowed his centre-right government would arrest the "criminals" behind the bombings.
Thursday's death toll was the biggest in an attack in Europe since December 1988 when a bomb exploded on board a Pan American Boeing 747, bringing it down on the Scottish town of Lockerbie. In all, 270 people died.