A court in Athens has found leaders of the Greek terrorist group November 17 guilty after a nine-month trial.
Giotopoulos said the guilty verdict was 'unfounded'
Nineteen members of the radical left group were accused of waging a campaign of bombing, robberies and at least 23 murders since 1975.
Among those targeted by the group were Greek public figures, businessmen and foreign diplomats.
They included the British defence attache, Brigadier Stephen Saunders, shot dead in Athens in June 2000.
Reacting to the verdicts of four defendants found guilty of the killing of Mr Saunders, his widow Heather said: "That's what we hoped for."
Athens Mayor and Olympic Games host Dora Bakoyiannis, whose parliamentarian
husband Pavlos Bakoyiannis was shot dead by the group in 1989, said: "Greek justice spoke today. Its decisions are respected by all."
A 20-year statute of limitations in Greek law means that group members will not be sentenced for the first four killings by November 17 - including that of CIA station chief Richard Welch, whose 1975 assassination marked the group's first appearance.
Sentences to follow
The man considered to be the group's leader, Alexandros Giotopoulos, has been found guilty of membership of the group and involvement in assassinations.
Giotopoulos, who denied links to the group, said: "The decision is unfounded."
As he was led from court he shouted: "Today's Greece is a modern colony of the United States."
Dimitris Koufodinas, whom prosecutors argued was the group's main hitman, was also found guilty of membership of the group.
The three judges, who heard the cases without a jury, have to read through hundreds of charges against the 18 men and one woman on trial.
So far, four of the accused, including the only female defendant, Angeliki Sotiropoulou, have been acquitted for lack of evidence.
Sentencing is not expected before Wednesday, with the ringleaders expected to receive multiple life terms.
The court had to reach a verdict within 18 months of the suspects' detention in prison, and that deadline was due to run out in January.
Greek police made little progress in their search for the group until last year, when they caught one of the accused apparently trying to plant a bomb.
He informed police of the names and whereabouts of other November 17 members.
Within weeks, police made a series of dramatic arrests.
Giotopoulos, who was charged with direct involvement in 10 attacks, could face 19 life sentences for murder.
The BBC's Tabitha Morgan in Athens says the Greek Government is keen to show the international community that Greece is capable of eliminating terrorist groups ahead of next summer's Olympic Games in Athens.
It will also hope to make political capital out of the successful conclusion of the case ahead of general elections early next year.
November 17 took its name from the date of the crushing of a student uprising in 1973 during Greece's 1967 to 1974 military rule. The radical leftists' attacks were aimed at overthrowing capitalism and attacked US targets because of Washington's support for the junta.
Brigadier Saunders' murder was the last carried out by the group.