Greek police have rounded up nearly 5,000 people in an operation targeting illegal immigration in Athens.
More than 1,130 people have been arrested in the capital since Saturday. More arrests are expected.
The crackdown comes amid fears the fighting in Syria could lead to a new influx of refugees.
More than 80% of migrants entering the European Union do so through Greece, which is in the grip of its worst recession in decades.
Some Greek politicians have called for the government to adopt a harder line on illegal immigration.
The BBC's Mark Lowen in Athens says police have targeted migrants across the Greater Athens area.
A police spokesman told the BBC that illegal migrants without the necessary paperwork would be repatriated.
About 200 Pakistanis were among suspected illegal migrants rounded up in Athens on Sunday, the Ekathimerini newspaper reported. They were to be sent home.
"We do not care about colour, ethnicity or religion of the illegal immigrants," Public Order Minister Nikos Dendias said on Saturday.
"The only criterion is the observance of laws with full respect, I repeat with full respect, for human rights and the European order."
Our correspondent says the issue of illegal immigration has shot to prominence in Greece over recent months after the far right Golden Dawn party won enough votes to enter parliament.
The Greek authorities have increased the number of guards at the border with Turkey amid fears there may be a sudden influx of refugees entering Greek territory as the situation in Syria deteriorates, he adds.
Greece has frequently called on other European nations to do more to help tackle illegal migration into the EU, arguing that it takes the strain.