Police raided several hideouts in Athens and its suburbs, confiscating weapons and explosives, and arresting at least 10 suspected terrorists.
Police said the counterterrorism operation was still under way, and they declined to identify the arrested suspects.
The confiscated material included three submachine guns, three AK-47 assault rifles, seven pistols, several rounds of ammunition, 200 grams of TNT, four hand grenades and 50 kilograms of ANFO, a powerful explosive mixture of ammonium nitrate and fuel oil widely used in improvised explosive devices, a police statement said.
"This is a case that the police have been following for some time now and the operation is still at its early stages," said police spokesman Athanasios Kokalakis. "All I can tell you is that we have found weapons and they are being examined by experts to see if they have been used in (terrorist) attacks."
Police would only identify one of the areas where the raids were occurring: the suburb of Nea Smyrni. But Greek media reported other sites as the center of Athens; the neighborhood of Exarchia, long known as a base for Greek anarchists; the port of Piraeus, southwest of the capital; and other sites within the Athens area.
Kokalakis declined to say which Greek terrorist organizations were being targeted, but two police sources told The Associated Press they were the Conspiracy Nuclei of Fire and the Sect of Revolutionaries. The sources spoke on condition of anonymity in keeping with police regulations.
Last month, Conspiracy Nuclei of Fire claimed responsibility fora spate of parcel bombings in Greece. Most of the 14 packages with small amounts explosives were intercepted by police and destroyed, but a delivery service employee suffered minor burns in one small blast. Two suspects were arrested and they eventually admitted membership in the group.