Greece's unelected government unveiled plans on Monday to build detention camps for 30,000 undocumented migrants by 2014.
Public Order Minister Michalis Chrysohoidis said 30 facilities - officially named "closed hospitality centres" - would be created on unused military sites under a €250 million (£207m) programme funded by the European Union.
The pledge was issued just weeks ahead of national elections expected by early May.
"Whoever enters the country will be given hospitality and will be immediately asked to return home," Mr Chrysohoidis told a press conference.
"Unless they are refugees in which case they have a sacred right to asylum."
The detained immigrants would far outnumber Greece's prison population, currently estimated at 12,500.
Greece is the busiest transit point in the EU for undocumented migrants, who cross the porous land and sea border with Turkey by the tens of thousands every year.
Greek authorities claim more than a million people are living illegally in the country.
"The current situation cannot continue," Mr Chrysohoidis said.
"Hundreds of thousands of people are wandering aimlessly through the streets, being forced to break the law, being exploited by criminal networks and deterring legitimate immigrants from staying in the country."
He said each detention centre would create 1,000 local jobs including up to 300 private security guards supplementing armed police.
The first new centre is due to open next month at an abandoned former army base in northern Greece, amid protests by local residents who say there is no time to build suitable facilities for so many inmates.