Amnesty International has said it is "appalled" by reports that Greek police tortured Afghan immigrants.
Greece has signed international agreements condemning torture
The Greek public order ministry is investigating claims that police abused the immigrants, including children, at a house in the capital, Athens.
At least 40 Afghans were reportedly detained by police hunting an Afghan immigrant who had escaped custody.
They alleged that police beat them, forced them to strip naked and staged mock executions.
According to Amnesty's Greek branch, at least one child aged 17 has come forward with allegations of torture.
The organisation claimed that about 60 Afghans were beaten, but only half that number actually lodged complaints.
At least 17 of the Afghans were aged 15 to 17, Amnesty said.
"Amnesty International condemns such ill-treatment in the strongest possible terms," Amnesty spokeswoman Marianna Tzeferakou said.
'Culture of impunity'
"These incidents are even more abhorrent when perpetrated against the most vulnerable groups in society like minors and people who have come to seek refuge from persecution."
In a statement, Amnesty said it had written to Greek public order minister Giorgos Voulgarakis calling for a swift and impartial investigation.
The organisation has long alleged that Greek police violate human rights with impunity.
Maria Kali, director of the Medical Centre for the Rehabilitation of Victims of Torture, described injuries sustained by the Afghans as consistent with torture.
She told the AFP news agency that she personally tended to five children aged 17, while others were treated by other charities.
Correspondents say Greek non-governmental organisations have frequently condemned the treatment of asylum-seekers and illegal immigrants by the police.
The country is a signatory of the Optional Protocol to the Convention against Torture, which requires countries to work to prevent acts of torture.