Powerful explosions have torn through three Madrid train stations during the morning rush hour, with latest reports speaking of 173 people killed.
The blasts hit during the morning rush hour
Near simultaneous blasts hit Atocha station in the centre of the Spanish capital and two smaller stations.
No group has admitted responsibility but Spain's government blames Basque separatist group Eta for the attacks which come ahead of Sunday's election.
"There is no doubt Eta is responsible," said Spain's interior minister.
"Eta had been looking for a massacre in Spain," the minister, Angel Acebes, said after an emergency cabinet meeting.
"Unfortunately, today it achieved its goal," he told a news conference, saying the security services were certain Eta was behind the blasts.
There was no warning ahead of the attacks. In total there were 10 blasts, Mr Acebes said, and police had defused three other devices hidden in backpacks.
The three explosions at Atocha happened at about 0630 GMT , destroying a train that was pulling into the station.
There were also four blasts in a street outside.
"People started to scream and run, some bumping into each other," Juani Fernandez, a civil servant who was on the platform, told the Associated Press.
"I saw people with blood pouring from them, people on the ground."
A 28-year-old man called Mariano, who was travelling on the train, told El Pais newspaper that the carriage behind his had been thrown up in the air.
As people scrambled out, he tried to help the injured.
"I held a girl as she died in my arms," he told the newspaper.
Atocha is a huge rail station used by commuter and inter-city trains, which also has a metro station.
There were similar scenes of destruction and chaos at two smaller commuter stations: El Pozo and Santa Eugenia.
From the initial confusion, the scale of death and destruction has become ever more apparent.
The authorities say that, so far, 173 people are known to have died.
At least 600 people have been injured and Madrid's hospitals, swamped with casualties, have made an urgent appeal for blood donations.
Spain's political parties suspended campaigning for Sunday's poll.
The BBC's defence correspondent Jonathan Marcus says that, in the absence of an Eta claim of responsibility, there will inevitably be continuing speculation about other potential perpetrators.
Spain's strong support for the US and Britain in the run-up to war with Iraq could make Spain a target in the eyes of shadowy Islamic groups, he says.
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Such a suggestion has already been made by the leader of the banned Basque separatist party Batasuna, who denied that Eta could have been behind the attacks.
Arnald Otegi pointed the finger instead at what he called "the Arab resistance".
Mr Acebes, speaking earlier from the scene of the blast at Atocha, said, "Have no doubt, those responsible will be caught and will pay for their crime."
The Basque regional president, Juan Jose Ibarretxe, stressed that Eta does not represent the Basque people.
"When Eta attacks, the Basque heart breaks into a thousand pieces," he said.
The Spanish government has declared three days of mourning for the victims and called on Spaniards to stage rallies on Friday evening to condemn the attacks.
If it is confirmed as Eta's work, it would be their deadliest attack in more than three decades of armed struggle for independence.
MAIN ETA ATTACKS
July 2003: Bomb attacks in Alicante and Benidorm, 13 injured. Further explosion at Santander airport days later
Jan, Feb 2000: Car bombs explode in Madrid and the Basque capital Vitoria
June 1998: Car bomb kills Popular Party councillor Manuel Zamarreno
July 1997: ETA kidnaps and kills Basque councillor Miguel Angel Blanco
June 1987: 21 shoppers are killed in an attack on a Barcelona supermarket
1980: In ETA's bloodiest year, 118 people are killed
Dec 1973: Assassination of Prime Minister Admiral Luis Carrero Blanco
Last month, two Basques suspected of being Eta members were arrested as they headed to Madrid in a truck laden with explosives.
Spanish police said they were arrested about 140km outside the city with 500kg of explosives hidden in the vehicle.
And last December, Spanish authorities said they foiled a Basque separatist plot to blow up a train at a Madrid rail station.
France has stepped up its police presence on its border with Spain in response to Thursday's attacks, the French news agency AFP reports.
Close co-operation between the two countries has led to dozens of arrests of suspected Eta members in southern France in recent years.
1. Atocha station, 0639 GMT: Four bombs explode on a train entering the station, 59 people killed. Three blasts on a train inside the station, 30 people killed
2. El Pozo station, 0641 GMT: Two explosions, 67 people killed
3. Santa Eugenia station, 0642 GMT: One explosion, 17 people killed