A Greek prosecutor ordered an investigation on Monday (local time) into whether four suspected bank robbers were beaten in custody, after police published mug shots that were altered to make their injuries appear less severe.
Rights groups and critics have long accused Greek police of detaining immigrants and other prisoners in shocking conditions.
Photos published in the Greek media of the men, who were aged between 20 and 25 and arrested on Friday, showed them bruised and bleeding while being escorted by police.
But mug shots released by the police over the weekend had injuries missing. One had been altered to remove a purple bruise from beneath the suspect's left eye. In another, black bruises below the suspect's eyes and cheeks appear to have been erased.
Public Order Minister Nikos Dendias acknowledged the photographs had been tampered with, but defended the decision by saying it was to make the four men recognisable to the public.
"Photoshop was used and I - just like you, just like any reasonable person - asked why was this done," Dendias said on Greek television when asked about the press reports.
"Because if they hadn't been Photoshopped, in order to make them resemble an image that the average person would recognise them in, then the photos wouldn't have been published in the first place."
He said police believed the injuries occurred when the suspects resisted arrest rather than when they were in custody.
The leftist Syriza opposition criticised Dendias's comments and demanded an investigation into allegations by the families of the men that they had been beaten in police custody.
Amnesty International backed calls for an impartial and thorough investigation, saying it was concerned by allegations of torture and ill-treatment by Greek police.
"The Greek authorities cannot just Photoshop their problems away. This culture of impunity needs to be stopped," said Marek Marczynski, deputy director of Amnesty's Europe and Central Asia Programme.
Greek news websites published a letter by the mother of one of the suspects accusing the police of mistreating her son because of his anti-establishment beliefs.
"Police authorities in Greece are following the examples of torture in Guantanamo prison," the letter said. "My son - and the others arrested - was not treated like every other law breaker but with particular hatred because he is an anarchist."
The four men were arrested during an attempted armed robbery at the branches of Greek lenders ATEbank and Hellenic Postbank in the northern town of Kozani on Friday.
Two of the detainees appeared in court on Monday in a separate case, on charges of belonging to Conspiracy of Fire Cells, an urban guerrilla group which has claimed responsibility for a spate of bomb attacks since 2009.
Greece has seen a surge in political violence in recent weeks, including makeshift bomb explosions outside the homes of journalists and political figures as well as shots fired at an office used by Prime Minister Antonis Samaras.