Epaminondas Korkoneas is accused of murdering a 15-year-old schoolboy
The mother of Alexandros Grigoropoulos - the Greek schoolboy shot dead in 2008 - has called the policeman who fired the fatal bullet a "monster".
Tzina Tsalikian told a court that Epaminondas Korkoneas deliberately shot her 15-year-old son, who had as much value to them "as a cockroach".
Mr Korkoneas, 38, denies murder and his colleague Vassilis Saraliotis, 32, denies complicity.
The shooting sparked weeks of riots across Greece.
The trial was moved from Athens to Amfissa - a small town 200km (120 miles) west of the capital - to deter attacks by anarchist groups which have vowed to kill the two defendants.
The fatal shooting precipitated the worst riots in Greece's recent history, and led to a loss of national confidence in the police as an institution.
Speaking at the trial in her first public comments since the shooting, Mrs Tsalikian said both defendants were "monsters in the guise of men".
She described her son as "a quiet child who trusted the police".
There is no dispute that Mr Korkoneas fired the shot that fatally pierced the schoolboy's heart in December 2008.
The prosecution says the policeman aimed directly at the boy, but the veteran police officer argues he had responded to youths throwing objects at his squad car by firing a warning shot which ricocheted fatally.
"I don't accept liability for anyone's death," he told the court on Friday.
The key forensic evidence will be the ballistic tests on the bullet, says the BBC's Greece correspondent Malcolm Brabant.
Important testimony is expected to come from the teenager's friends who were with him in the rebellious Athenian district of Exarchia when the shooting took place, our correspondent adds.
The boy's family have been hurt by accusations made shortly after his death that Alexandros was a troublemaker.
Passage of justice
Both the boy's family and the main police union have objected to the trial taking place so far from Athens.
The family believes the distance will make it difficult for key witnesses to attend. The police union regards the move as an insult that implies officers could not guarantee security in the Greek capital.
The 2008 riots raged for more than two weeks
The Greek police union believes the trial is a vital process in rebuilding trust between civilians and the force.
Union president Christos Fotopoulos said society had to be convinced that there had been a fair passage of justice.
"If the public believes there is a fair and proper trial, and the defendants receive proper justice, then this will improve relations between the police and society," he told the BBC.
The restricted number of access roads into Amfissa has given the authorities a chance to prevent large numbers of potential rioters from getting through.
But shopkeepers in the town have pulled down the shutters in fear that they will bear the brunt of anti-establishment wrath.
The trial, which was adjourned after several hours of testimony and is set to resume next week, is expected to last several months.