Greek authorities on Monday faced an uproar after allegations that police had brutally beaten four suspected members of a local terrorist group before publishing manipulated mugshots of the men in a crude attempt to disguise their injuries.

A prominent Athens prosecutor ordered an investigation after relatives of the men – aged between 20 and 24 – accused Greek police of adopting "Guantanamo- style" tactics.

There was a public outcry after the media published doctored pictures of the suspects who were arrested on Friday after attempting to rob two banks in the crisis-hit north. Despite digitally altering the photos the detainees bruised eyes and lips were evident.

Promising that instances of torture, would not be tolerated, the Greek public order minister, Nikos Dendias, said "punishment will be merciless".

"There is no desire to cover [up] anyone for anything," he told the private TV channel Mega. Earlier, the public order ministry had issued a statement describing the detainees as being especially dangerous.

"They [had] threatened human lives with Kalashnikovs and they had taken a hostage," it said. "Armed engagements don't happen with the exchange of flowers, the accused had … resisted arrest."

All four are believed to be linked to politically extreme militant groups that have arisen on the right and left in Greece in the past three years, as the countryhas been increasingly hobbled by economic crisis and social upheaval.

Two of the detained men, Yannis Michailidis, 24, and Dimitris Politis, 21, were said to be members of the urban guerilla group Conspiracy of the Cells of Fire and were tried in absentia.

The gang gained notoriety in 2010 when it claimed responsibility for a series of mail bombings targeting foreign embassies and European officials. German chancellor Angela Merkel's office in Berlin was one of those attacked although no one was injured in the blasts.

A third suspect, whose picture was printed in newspapers on Monday, is Nikos Romanos, 20, described as the best friend of Alexis Grigoropoulos, the teenager shot by police in December 2008. His death led to widespread violence that shocked the nation and its European partners.

Like his friend, Romanos, who was present on the night of the shooting, comes from a family of businessmen and writers in Athens's wealthy northern suburbs. On Monday he described himself as a "prisoner of war." Police claim his fingerprints had been found in a safe house used by the Conspiracy of the Cells of Fire.

Dendias said the mug-shots had been released by police, despite digital doctoring, so that the suspects would be "recognisable to the public" and authorities could conduct further investigations into possible hide-outs they had used.

An anonymous tip-off led police to a safe house rented by some of the suspects in the Athens suburb of Maroussi on Monday.

But the crude Photoshop methods used to tone down the injuries the men had sustained has unleashed political outrage including opposition from the radical left Syriza party. Describing the methods as a "disgrace," the party reminded Greeks of the Guardian's exposure last year of similar cases of brutality.

"Mr Dendias' statements are further evidence of the right-wing turn of today's government which is targeting… rights, freedoms and finally democracy itself," the Syriza party said and it called for a thorough investigation.With the government clearly on the defence even the small Democrat Left party, which is supporting it, felt fit to decry the police tactics. "Terrorism is the enemy of society and democracy must confront it decisively… but democracy does not exact revenge. An investigation and explanation are required as regards the faces that have clearly been deformed due to the brutality used."