Greece suspended overseas shipment of mail and packages for 48 hours on Wednesday, hoping to stop militants sending more parcel bombs in addition to more than a dozen already sent to foreign governments and embassies.
Small bombs exploded at the Swiss and Russian embassies in Athens on Tuesday, a parcel with explosives was intercepted at the German chancellor's office and another package addressed to Italy's prime minister caught fire when it was checked.
The bombs may be intended to spur an anti-government vote in Sunday's local elections in protest against Prime Minister George Papandreou's austerity plan, agreed with the EU and International Monetary Fund to deal with Greece's debt mountain.
"Such brainless and irresponsible actions aim to damage the Greek people's great effort to put the country and its economy back on its feet ... they will fail, we will not bend," said Papandreou, who has vowed to be "merciless" with militants.
Papandreou, in office for only 13 months, and acutely aware of the disruption caused by weeks of riots in December 2008 after a police officer killed a teenager, has threatened to call a parliamentary election if voters do not back him.
There have been a total of 14 actual or suspected bombs since Monday but only one casualty, a courier employee who was slightly injured when a bomb exploded in her hands.
"If there are more parcel bombs they must be abroad by now, not in Greece. Courier services have made checks and have not found anything suspect for domestic delivery," a police official said on Wednesday.
Police, who said their inquiry had shown no links between the bombers and al Qaeda, said members of the public had come forward with information about two Greek men who have been arrested, while investigators searched their homes.
"All evidence show this is a clear domestic case, with no connection with international terrorism," Greek Foreign Minister Dimitris Droutsas said. "The evidence so far shows we are dealing with extreme left, anarchist groups."
The Greek parcel bombs follow the interception of explosive packages suspected to have been sent to Chicago by al Qaeda in the Arabian Peninsula.
German officials said on Wednesday they want the EU to introduce new security measures against militant attacks via air freight to help improve a patchwork of rules around the world.
Fire conspiracy cells
Greek police arrested the two men, aged 22 and 24, in possession of two bombs, one addressed to French President Nicolas Sarkozy.
One of the two arrested men is suspected of being a member of the Fire Conspiracy Cells, who have staged several attacks on government targets over the past year, police said.
Two suspect packages detonated by police at the cargo terminal of Athens airport on Tuesday contained explosives and were addressed to European police organization Europol and the European Court of Justice, police officials said.
Greece has seen anti-capitalist attacks down the decades, and has been hit by gas canister and bomb attacks claimed by leftist militants since the 2008 riots, the worst in decades.
The attackers may want to show that they are active after the arrests of more than a dozen suspected guerrillas this year.
"The new terrorists, and those that possibly instruct them, aim at creating a negative image for the country abroad," said the liberal Ta Nea newspaper in an editorial, reflecting concern that Greece will be tagged as a country prone to violence.
"The absurdity of terrorism could threaten the painful efforts our country is making to stabilize its economy and improve its international image," Greek President Karolos Papoulias said in a statement.
Any sense of political instability or wider social unrest could drive up the Greek government's borrowing costs and further strain its attempts to stabilize its finances.
Papandreou said he would "be merciless to those who attempt in vain to rock social peace with terrorist acts and hurt our country's image internationally during a very difficult period."
The two arrested men were both charged with participating in a terrorist group and with illegal possession of explosives and weapons, a court official said.
No one was injured at the Swiss or Russian embassies, and Greek police said the bombs were generally too small to kill.
Other parcel bombs found on Tuesday included one outside parliament addressed to the Chilean embassy, one each at the Chilean and Bulgarian embassies, and one addressed to the German embassy, at the offices of a courier company.
In Germany, police found a package containing explosives sent from Greece to Chancellor Angela Merkel's office in Berlin.
In Italy, a cargo plane from Athens was forced to land in Bologna after authorities were told it was carrying a suspect parcel addressed to Prime Minister Silvio Berlusconi. The package caught fire as it was checked.
Greece's stock market took the bombs in its stride. Shares closed little changed on Tuesday and were flat on Wednesday.