At least 11 mail bombs were detected in the Greek capital on Monday and Tuesday -- one addressed to French President Nicolas Sarkozy and eight to the Athens embassies of Bulgaria, Russia, Germany, Switzerland, Mexico, Chile, the Netherlands and Belgium.
Three of the bombs exploded or caught fire in Athens, causing minor damage and leaving one delivery service employee burned.
Police investigators said none of the devices examined so far contained lethal amounts of explosives. But the attacks highlighted the difficulty of keeping bombs out of the international delivery system -- also a target of Yemen-based militants armed with more powerful and potentially deadly explosives.
Authorities are questioning two suspects arrested Monday in connection with the bombings, and released the photographs of five other suspects believed to be associated with them. The suspects, most in their early 20s, have been linked to an anarchist militant group called Conspiracy Nuclei of Fire.
Greece has suffered a spike in militant attacks -- including a deadly letter bombing earlier this year -- since massive riots in 2008 triggered by a police shooting of a teenage boy.
Prime Minister George Papandreou said the government would be "unyielding" in its pursuit of bombers.
But terrorism experts say Greek police face a difficult task in trying to shut down the mail bombing campaign.
"It's very difficult to prevent the initiation of this kind of attack ... The initiative lies with the people who want to mount the attack, not with the authorities," said Michael McKinley, a senior lecturer in International Relations at the Australian National University in Canberra.
McKinley said fully screening all mail would "bring the Greek postal system to a halt" while catching the five remaining suspects might not stop the problem.
"It depends how many committed people are following them," he said. "I most cases of homemade explosives, the people involved are not very expert. So effectively this problem when you have an open society: If people are determined to cause a problem they can do."
Also Wednesday, police said 11 cars were burned in overnight arson attacks, most using small cooking gas canisters, including eight vehicles owned by the state electricity company.