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. Also on Friday, a US court will hear a guilty plea from the only man charged in the US over the attacks. Zacarias Moussaoui, who was born in France to Moroccan parents, has been charged with conspiracy. He could face the death penalty if convicted. The defendants in the Spanish trial 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 main defendant at the trial is Syrian-born Immad Yarkas, 42, alias 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. 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. Mr Yarkas has denied that he recruited Islamic militants or held planning meetings.
Another suspect, Ghassub al-Abrash Ghaylun, allegedly filmed the World Trade Center and other US targets. The tapes were allegedly passed on to "operative members of al-Qaeda and would become the preliminary information on the attacks against the Twin Towers", wrote Judge Garzon in the indictment. 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 others appearing in court on Friday are charged with membership of al-Qaeda. They include a journalist from the Arabic TV station al-Jazeera, Tayseer Alouni, who interviewed Bin Laden after the attacks. He is accused of using a posting in Afghanistan to distribute money to the militant Islamic network. All the suspects deny the charges. Mr Yarkas' lawyer, Jacobo Teijelo, said Spanish prosecutors "have no solid evidence of anything". He said if Mr Yarkas and his alleged accomplices were plotters, the US would have sought their extradition, the Associated Press news agency reported. "That is jarring from the point of view of common sense," AP quoted Mr Teijelo as saying.