The police said the raid, carried out with the assistance of Greek intelligence officers, had targeted two flats in the Athens districts of Sepolia and Exarchia.
The police said they had found "a tunnel and heavy weaponry".
"Over 20 foreigners" had been detained for questioning, the police said.
According to reports, the detained are suspected of belonging to the Revolutionary People's Liberation Army-Front (DHKP-C), a far-left group active in Turkey.
The DHKP-C has carried out sporadic attacks over the years and is listed as an extremist group by Turkey, the European Union and the United States.
Nine people had been arrested in Greece in 2017 ahead of a visit by Turkish President Recep Tayyip Erdogan.
They were initially charged as DHKP-C members and jailed for months before being released last year after convincing a court that they were political refugees.

Copyright Agence France-Presse, 2020