Three men have appeared in court in Spain charged with helping to plan the 11 September 2001 attacks in the US.
Immad Yarkas is alleged to have lead an al-Qaeda cell in Spain
They include the alleged head of an al-Qaeda cell in Spain, who is accused of arranging a meeting allegedly attended by one of the 19 attackers.
Another man is charged with filming the World Trade Center and giving the tapes to militants before the US attacks.
They are among 24 defendants in Europe's largest trial of suspected al-Qaeda members. All deny the charges.
As the 24 arrived at the court in Madrid, armed police stood guard outside the building and a police helicopter circled overhead.
The suspects - mostly men born in Syria or Morocco - sat in a bullet-proof chamber as the charges were read out.
During the opening hearing prosecutors questioned Jose Luis Galan, also known as Ghassub al-Abrash Ghaylun.
Mr Galan is accused of filming the World Trade Center and other US targets.
The tapes were then allegedly passed on to "operative members of al-Qaeda and would become the preliminary information on the attacks against the Twin Towers", the indictment said.
"I absolutely condemn all terrorist acts, all violent acts," the Spaniard told the court, Reuters news agency reported.
The main defendant at the trial is Syrian-born Immad Yarkas, 42, known as Abu Dahdah.
Mr Yarkas was arrested by Spanish authorities in November 2001 on suspicion of heading an al-Qaeda cell that allegedly provided funding and logistics for the people who planned the attacks on the World Trade Center and the Pentagon.
Along with Moroccan-born Driss Chebli, he is said to have set up a meeting in June 2001 which was allegedly attended by at least one of the attack ringleaders, Mohamed Atta, who piloted one of the planes that hit the Twin Towers.
Prosecutors want the three men to receive more than 60,000 years in jail - 25 for each of the more than 2,000 people killed on 11 September 2001.
The rest of the defendants are charged with belonging to a terrorist group.
Jose Luis Galan - Allegedly filmed New York landmarks in 1997 for al-Qaeda. He is accused of 2,500 murders and belonging to a terrorist group
Immad Yarkas - Al-Qaeda's alleged leader in Spain, he is accused of 2,500 murders, belonging to a terrorist group and possessing counterfeit money
Driss Chebli - Allegedly helped al-Qaeda members involved in the 9/11 attacks. He is accused of 2,500 murders and belonging to a terrorist group
21 other defendants - Face charges including membership or association with a terrorist group, weapons possession, falsifying documents and fraud
All have said they are not guilty of the charges
They include a journalist from the Arabic TV station al-Jazeera, Tayssir Alouny, who interviewed Bin Laden after the attacks.
Mr Alouny, who was granted temporary bail for health reasons on Monday, is accused of using a posting in Afghanistan to distribute money to the militant Islamic network.
All the defendants are part of a group of 41 suspects indicted by the anti-terrorist judge Baltasar Garzon.
Judge Garzon says Spain was a key base for hiding, helping, recruiting and financing al-Qaeda members in the lead-up to the attacks on New York and Washington.
The group also includes Osama Bin Laden and other senior figures in al-Qaeda, but under Spanish law suspects cannot be tried in absentia.
The trial will resume on Monday.