Amnesty International has urged Greece to stop treating asylum seekers as criminals and holding them in detention centres.
In a new report, the human rights organisation says it is particularly concerned about unaccompanied children who end up in detention centres.
This is the latest in a series of international reports damning Greece's treatment of illegal immigrants.
Greece is the main door to Europe for refugees and economic migrants.
Nicola Duckworth, Amnesty's programme director for Europe and Central Asia, said asylum seekers and irregular migrants were not criminals. Yet, the Greek authorities treated them as such, disregarding their rights under international law.
Currently, migrants were detained as a matter of course, irrespective of whether such a measure was necessary, she said.
Avlona detention centre north of Athens, which is supposed to house young Greek criminals, typifies the situation described by Amnesty.
It contains 360 teenage boys, who travelled alone from Sudan, Iraq, Afghanistan and Somalia.
One former foreign inmate said the boys were locked up four to a cell for 15 hours a day.
The detention centre is infested with cockroaches, and conditions are so spartan that inmates are not given toilet paper, toothpaste or razors.
They rely on charity for essential supplies, or work for other prisoners to earn money to supplement their meagre rations.
Greece has promised to improve conditions at so-called reception centres and to improve the slow asylum application procedure.
But, facing an influx of tens of thousands of migrants each year, it is struggling to meet the standards demanded by its foreign critics.