Parcel bombs have exploded at the Swiss and Russian embassies in the Greek capital, Athens, and several suspicious packages have been destroyed.
Two parcel bombs were destroyed in controlled explosions at Athens airport's cargo terminal.
A suspected bomb was destroyed at the Bulgarian embassy and another, posted to the Chilean embassy, in a van.
Greece says it is halting international air mail for 48 hours to allow for further investigations.
No-one was hurt in the Athens blasts, which came a day after four parcel bombs were found in the city.
The BBC's Malcolm Brabant in Athens says it appears to be a co-ordinated attack by an extreme left-wing group.
The first explosion happened at the Swiss embassy.
First reports said the device had been thrown into a courtyard, but police later said it had been left at the embassy's entrance.
"When the external packaging was removed, the contents burst into flames," a police spokesman said.
Bomb squad officials were on their way to the Russian embassy when the device there exploded.
Police carried out controlled explosions at Athens airport on packages addressed to International police organisation Europol in the Netherlands and the European Court of Justice in Luxembourg.
The Bulgarian embassy was sealed off as experts carried out a controlled explosion on a suspected parcel bomb.
Another suspected device addressed to the Chilean embassy was found in a delivery van outside the Greek parliament and destroyed.
The courier carrying the parcel was worried and alerted police on guard outside the building.
Officials said another suspected bomb was intercepted at the offices of a courier company addressed to the German embassy.
German officials said a parcel bomb sent to Chancellor Angela Merkel's office had come from Greece.
"It seems this is a continuation of yesterday's attacks and that Greek guerrillas are behind it, but we are still investigating," police spokesman Thanassis Kokkalakis told Reuters news agency.
On Monday, a parcel bomb addressed to the Mexican embassy in Athens blew up at a courier office, slightly injuring an employee.
Police later arrested two suspects and found two more bombs, one addressed to French President Nicolas Sarkozy and the other to the Belgian embassy.
A fourth bomb was found at a delivery company addressed to the Dutch embassy.
Terrorism experts suspect the co-ordinated campaign is the work of a group called the Conspiracy of the Cells of Fire, which is trying to spark revolution in Greece during the current period of austerity.
The parcel bombs have raised anxiety levels in Greece in the run up to this weekend's vital local elections, our correspondent says.
The elections are seen as a referendum on the socialist government's handling of the economic crisis, and Prime Minister George Papandreou has warned he may call a general election if his party is soundly defeated.