Spain is beginning three days of mourning for the deaths of at least 198 people in bomb attacks in Madrid.
Many thousands are expected to join nationwide protests
More than 1,400 people were injured as 10 bombs ripped through commuter trains during Thursday morning's rush hour.
Schools, museums and the Central Bank will be shut, and huge crowds are expected throughout Spain for peaceful demonstrations.
Lines of investigation are being kept open after clues apparently implicated either Basque or Islamic militants.
In an address to the nation on Thursday, King Juan Carlos said "terrorist barbarity" had plunged Spain into the deepest grief.
Newspapers are describing the attacks as "Spain's 9/11" and parties have halted campaigning for Sunday's general election.
Spanish Foreign Minister Ana Palacio said everything appeared to implicate Eta in the attacks, with "very strong clues" and "very strong precedents" backing that view.
The group has previously targeted the Spanish railway system and two Eta suspects were arrested last month driving a truck loaded with more than 500kg of explosives headed for Madrid.
But Ms Palacio said other possibilities could not be ruled out. On Thursday the interior minister said a stolen van had been found near the route of the trains that contained seven detonators and a tape recording of Koranic verses.
0639 GMT: Four bombs explode on a train approaching the Atocha terminus, killing at least 59
Almost simultaneously, three bombs detonated in Atocha station killing 30
0641 GMT: Two explosions on a double-decker train kill at least 70 people
0642 GMT: Bomb rips through a train at Santa Eugenia station killing some 17 people
A London-based Arabic newspaper, Al-Quds, said it received an e-mail in which a group linked to al-Qaeda claimed to have carried out the attacks.
The message purportedly from the Abu Hafs al-Masri Brigades said it had attacked "America's ally in its war against Islam" on behalf of al-Qaeda.
But American intelligence officials have cast doubt on the claim, saying al-Qaeda does not usually claim responsibility so early.
Officials said there were no plans to raise the colour-coded threat level of terror attacks in the US, which currently stands at yellow - an elevated state of alert.
Spanish editorial writers are demanding answers before voters go to the polls, because the culprits' identity might influence people's choice of party.
The ruling Popular Party campaigned on a hardline stance against Eta, but it also defied popular opposition by supporting the US-led war against Iraq - which may have triggered an attack by al-Qaeda.
Adding to fears that it may have been al-Qaeda, Madrid was struck exactly two and a half years, after the attacks in New York and Washington.
Messages of solidarity have poured in from around the world with President Bush saying America "stood strong" with the people of Spain.
Russia's Vladimir Putin called for "the entire international community" to unite against terror.
It was the worst terror attack in modern Spanish history and the deadliest in Europe since the Lockerbie airliner bomb killed 270 in 1988.
The harrowing task of identifying dozens bodies is continuing more than 24 hours after the blast.
Forty experts have drafted in and there has been a steady stream of grieving relatives at a large convention centre that is being used to house the dead.
The main Basque city, Bilbao, saw a large rally led by nationalists to condemn the bombs.
There were pot-banging protests in Barcelona and candle-lit vigils in Madrid.
The government said Friday's nationwide rally, due to begin at 1900 (1800 GMT), would show solidarity "with the victims, with the constitution and for the defeat of terrorism".