Human Rights Watch has called on Greece to stop its massive round-ups of migrants which it said was based on "little more than their physical appearance".
The New York-based charity warned that the crackdown on suspected illegal immigrants, which resulted in more than 6,000 people held for questioning by police in just over a week, has violated international standards.
More than 1,500 illegal immigrants were arrested for illegal entry and are poised to be deported, Greek police said.
"Greece has the right to enforce its immigration laws and after a fair process to deport people with no legal basis to stay in the country," said Benjamin Ward, deputy director of the Europe and Central Asia division at Human Rights Watch. "But it doesn't have the right to treat people like criminals or to presume irregular immigration status just because of their race or ethnicity."
The charity warned that mass expulsions were forbidden under international law.
Greece's public order minister, Nikos Dendias, defended the crackdown, ironically named after the Greek God of hospitality Zeus Xenios, saying the country's economic plight meant it could not afford an "invasion of immigrants".
Left-wing opposition parties have criticised the mass expulsions but far-right Golden Dawn party, which is standing on a platform of blatant xenophoboia, is likely to welcome the move.
Human Rights Watch criticised police inaction in tackling the issue of racist violence in Greece. The charity said assaults had reached "alarming proportions, with gangs regularly attacking migrants and asylum seekers".
"Greek police have a duty to protect all foreigners from violence, just as they do Greek citizens," Ward said. "These sweeps are a dangerous distraction from the real policing challenges the country faces."