Jose Maria Aznar, the Spanish Prime Minister, immediately blamed Eta, the Basque separatist guerrillas, after yesterday's bomb attacks.
Mr Aznar will feel vindicated whether the bombing was home-grown, Basque terrorism or al-Qa'ida.
The centre-right premier, in a televised address, said: "March 11 2004 has taken its place in the history of infamy."
Mr Aznar has adopted a tough stance against Eta during his two terms in office. He ruled out negotiations with the outlawed group, which authorities have blamed for the attack, and called for nationwide protests against Eta on Friday. "There are no negotiations possible or desirable with these assassins that have so often sown death through all of Spain," he said.
But Mr Aznar's certainties were undermined shortly afterwards, when Angel Acebes, the Interior Minister, announced that police investigating the attacks found a van with detonators and an Arabic-language tape with Koranic verses.
Julen de Madariaga, who created Eta in the 1950s and was once its leader, said it would be unlike the group to attack busy, working-class areas. But the ruling Popular Party leader, who is due to step down after a general election on Sunday, said "We will defeat them [Eta) ... We will succeed in finishing off the terrorist band, with the strength of the rule of law and with the unity of all Spaniards."
The government declared three days of official mourning. Mr Aznar called on Spaniards to take to the streets across Spain to demonstrate on Friday under the slogan, "With the Victims, With the Constitution and For the Defeat of Terrorism", to mark a day of attacks that has numbed and bewildered the country.
In the hours after the blasts, tens of thousands of people took to the streets across Spain in a spontaneous outpouring of pain and grief.
In Madrid, the capital, bars and shops stood empty throughout the morning as thousands of people gathered in the city's central Puerta del Sol square in a silent act of commemoration. In an expression of outrage, locals brandished signs condemning, "Eta assassins", and demanding, "Death penalty now".
Raul Martinez, a taxi driver, said: "We should take 300 Eta members out of prison now and execute them in the Plaza Mayor so everyone can see, just like people used to do."
Rosa Blazquez, a real estate agent, told The Independent in a telephone interview: "You feel the burden of being in the house on a day like this.
"My legs just took me out on to the street and led me down to the Puerta del Sol to be with other people.
"People from all over the world, including visitors from Europe, and Latin American immigrants, had all gathered there in a silent act of commemoration. There were no speeches or placards, and [the people] were just finding a way to deal with this."
In Barcelona, police said more than 7,000 workers and students joined four demonstrations held in different parts of the city, including one gathering at the emblematic Sant Jaume Square, attended by regional premier Pasquall Maragall.
Thousands of people, in cities as far apart as Cordoba and Granada in Andalusia and Zaragoza in northern Spain, broke with their routine to join demonstrations during the day, as people spoke of a need to share their grief at the terrorist attacks.
Laura Inclan, the Spanish actress, said: "The mood is very much one of shock and mourning. Everyone is going to the demonstrations, and all the stage shows in Madrid have been cancelled for today and tomorrow."
Leading trade unions the CCOO and UGT called for a 15 minute stoppage at factories and workplaces to mark the blasts, which ripped through commuter trains packed with workers travelling to their jobs from blue-collar commuter towns to the south of the city.
Ms Inclan, who queued for several hours yesterday to give blood at the city's Gregorio Maranon hospital, said: "It is very important to show our grief and turn out for the demonstrations tomorrow. We need to put our party politics behind us and show the world that democracy is the way ahead. There is no reason for this kind of act in Spain."
The government ordered three days of national mourning. Spanish footballers with the Real Majorca team, playing a UEFA cup match against Newcastle, wore black armbands for the match.
MADRID: CITY WITH A STEEL UNDERBELLY
This is a city of alegría, the city that never sleeps, whose roaring in-your-face spirit has delighted outsiders for centuries.
Its football bestrides the world. Its sunshine, its unique fast-food tapas culture and its art treasures combine to make Madrid widely considered as one of the most fun cities in the world.
But Madrid, whose sporting, artistic and avant-garde spirit has blossomed in the nearly 30 years since Franco, the former Spanish leader, died has a less-known, darker side. Intermittent terrorist attacks have buffeted the city throughout recent years, plunging Madrid into momentary extremes of revulsion and grief.
But it is in the nature of this resilient people that they soon recover and carry on with life as normal.
This latest atrocity stands comparisons with Franco¿s bombardments or the ravages of Napoleon's armies as a wound to the body and soul of the city, but they still do not do justice to the impact of yesterday's tragedy.
In the nine years I have lived in Madrid, the Spanish capital has become as fast and sophisticated as any in Europe, without losing its fierce distinctive identity. Once uniformly white, it is now as racially mixed as most other European capitals. The bus, tube and trains are a triumph of efficiency and a pleasure to use, although people still pile into their cars with manic enthusiasm, causing much exasperation.
This is a city so vivacious that people engaged in a normal conversation sound as if on the point of launching into furious combat. But beneath its carefree gaiety lurks a gritty, steely underbelly and an intensely emotional response to pain and tragedy.
Elizabeth NashReuse content