Athens police detained 501 people on Friday in an operation authorities say will be repeated "on a daily basis" to combat undocumented migrants and illicit trade.
The majority of those detained by the 455 police who took part in the sweep were foreign nationals.
In addition, 21 people were reportedly arrested on Thursday, nine of them migrants lacking legal documentation.
Citizen Protection Minister Michalis Chrysohoidis had announced the "clean-up operation" earlier in the week, as well as a plan to build detention centres for 30,000 illegal immigrants by 2014 with European Union funding.
The Communist Party of Greece has branded the detention centres "concentration camps," noting that while armed police will be responsible for external security the facilities themselves will be staffed by private security personnel.
It predicted on Thursday that "the clean-up operations, the mass arrests and deportations of immigrants will be accompanied by the intensification of intimidation and repression, attacking the rights of both local and foreign workers."