More than 600 witnesses and 107 experts have been called to testify in what prosecutors say was an al-Qaeda-inspired attack.
The defendants will give evidence from a bullet-proof box in the courtroom, which is in a trade fair pavilion because the premises of the National Court were deemed too small.
Basque separatists blamed
The prosecuting judge who prepared the case has linked the bombings to a call by Osama bin Laden to attack countries that supported the US-led war in Iraq and an essay that appeared on the internet urging attackers to hit Spain before elections that took place three days later.
|Nearly 2,000 people were injured in the blasts |
on four passenger trains [AFP]
An almost 100,000-page report drawn up by the state prosecutor and seen by Reuters news agency claims four men heeded the al-Qaeda call and started planning the attack in 2003. They recruited the others from criminal circles and a Madrid mosque.
Rabei Osman Sayed Ahmed, who is charged with inciting people to carry out the attack, will be the first to take the stand.
Ahmed, known as "Mohamed the Egyptian", has already been convicted of belonging to a terrorist group and sentenced to 10 years in jail by an Italian court. He has been extradited to Spain for the Madrid hearings.
'Difficult to prove'
Luca D'Auria, who defended Ahmed in Italy and is part of his legal team in Madrid, said: "It will be very difficult to prove that he had any responsibility for the attacks.
"The evidence against him was collected in Milan and there is no proof that he had any contact with the organisers, with the others accused of the attacks."
Ahmed is one of seven men facing sentences of about 40,000 years each for the deaths and membership of a terrorist organisation, although under Spanish law the longest jail term anyone can actually serve is 40 years.
Sarhane ben Abdelmajid Fakhet, alias "the Tunisian", who prosecutors believe "led and co-ordinated" the attacks, was one of seven men who died after blowing themselves up during a police raid on an apartment in the Madrid suburb of Leganes three weeks after the attacks.
The hearings are expected to last until July when the three-judge panel will retire to consider the evidence. They are not expected to come out with their verdicts and sentences until October at the earliest.