Greek authorities are under pressure to explain police pictures of four suspected bank robbers, three of whom are also suspected of being members of a domestic terrorist group, who appear to have been brutally beaten in custody.

The four men, aged 20 to 24, were arrested on Friday in northern Greece shortly after a bank robbery. Police released mugshots and asked for further information from the public.

All four photos appear to have been digitally manipulated, but signs of bruising show through the alterations on the left side of three suspects' faces.

The public order minister, Nikos Dendias, promised "merciless punishment" if it was confirmed that the four men had been tortured. "Justice will decide," he said in an interview aired on Mega TV. "There is no desire to cover anyone for anything."

But in a move that quickly provoked criticism, Dendias defended police tampering of the pictures saying the use of Photoshop methods was necessary to ensure the suspects were "recognisable".

Relatives of the four men said there was clear evidence they had been badly beaten while in custody.

Police in northern Greece issued a statement saying that only an "appropriate amount" of violence was used in restraining the suspects during their arrest.