Security forces were on alert across the Spain for the verdict in the 11 March 2004 Madrid train bombing trial.
The accused arrived at the National Audience, Spain's top anti-terror court, in a convoy of police buses.
Bomb-sniffing dogs and police helicopters were deployed around the court.
Judge Javier Gomez Bermudez read the much-awaited decisions of the three-judge panel, which came after a four-month trial.
The accused sat behind a glass wall under police guard as the verdict was read out.
Victims comforted each other during proceedings, with a hush falling over the courtroom as the verdicts were delivered.
Mohamed Moussaten was one of seven people acquitted. Twenty-one others were found guilty of charges including murder, forgery and conspiracy to commit a terrorist attack.
The president of the 11 March Association for the Victims of Terrorism, Pilar Manjon, condemned the decision, calling it a "mistake". She said her group would launch an appeal.