The 30 men, mostly Algerians, have been charged with membership of a terrorist organization, conspiracy to commit a terrorist attack and forgery.

The alleged mastermind Abderrahmane Tahiri, alias Mohamed Achraf, was extradited from Switzerland in April 2005.

Spanish authorities suspect Tahiri planned to ram a truck loaded with 1,100 pounds of explosives into the National Court in downtown Madrid.

"This was an organized and structured terrorist group, uncovered in November 2003, with radical Salafist tendencies, which defended the jihad (holy war) and intended carrying it out in Spain through violent actions such as that planned against the National Court and the persons within," according to the indictment.

"With that explosion, they hoped to kill the persons within (judges, clerks and public in general) and destroy the files held against the 'mujahedeen brotherhood' inside," the indictment said.

Investigating magistrate Fernando Grande-Marlaska said such an attack could have killed up to 1,000 people.

The prosecution is demanding sentences of between two and 46 years for the accused.

The trial is expected to last several months.

Police uncovered the alleged plot with the help of an unnamed informant who had lived with some of the accused.

In an initial investigation, Spanish Judge Baltasar Garzon claimed Tahiri set up a cell known as the "Martyrs for Morocco" while he served time in a Spanish prison for credit card fraud between 1999 and 2002.

Garzon said the cell had links with other Islamic terrorists, including the group believed to be behind the March 11 train bombings in Madrid that killed 191 people.