The head of the radical Greek November 17 organisation has been jailed for life, as has the group's main hitman.
Giotopoulos led the group and Koufodinas was its main hitman
Alexandros Giotopoulos and Dimitris Koufodinas were among 15 people found guilty last week of involvement in a 25-year series of crimes.
The group's victims included Greek and foreign figures, among them a CIA station chief and a UK defence attaché.
Mr Giotopoulos, 59, who was found guilty of planning the group's actions, had denied links to November 17.
He is expected to appeal.
A three-judge panel spent nine months hearing evidence related to some 2,500 crimes including murder and robbery.
Multiple life terms
It was Greece's first terrorism trial.
Mr Giotopoulos was sentenced to 21 life terms - the longest in Greek legal history - for his role in plotting 19 murders.
Mr Koufodinas was given 13 life terms. Four other members of the group involved in killings were sentenced to as many as 10 life terms.
The maximum amount of time any of them will spend behind bars, though, is 25 years, the BBC's Richard Galpin in Athens says.
They will be eligible for parole after 20 years.
Greece does not have the death penalty.
Greek police had 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.
The BBC's Tabitha Morgan in Athens says the Greek Government is keen to show the international community that Greece is capable of eliminating such 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 military rule in Greece which lasted from 1967 to 1974.
Brigadier Saunders was the last person killed by the group
The radical leftists' attacks were aimed at overthrowing capitalism and attacked US targets because of Washington's support for the junta.
They are believed to have killed four US officials, two Turkish diplomats and a UK military attache in addition to their Greek victims.
A 20-year statute of limitations in Greek law means that group members will not be sentenced for the first four killings blamed on November 17 - including that of CIA station chief Richard Welch, whose 1975 assassination marked the group's first appearance.
Brigadier Stephen Saunders, who was shot dead as he drove to work in Athens in June 2000, was the last person killed by November 17.
His widow, Heather Saunders, of Melbury Osmond, Dorset, said last week's guilty verdict was one her family "had hoped for".
"They killed 23 people," she told the BBC. "But it is 23 widows, there's goodness knows how many children, how many parents; they will all carry this scar for the rest of their lives."