Greece has suspended international airmail for 48 hours after more than a dozen suspicious packages were sent to targets in Greece and across Europe.
Police are looking for five men suspected of being part of an extreme left-wing group.
German Chancellor Angela Merkel has called for improved security for air cargo after one of the packages was sent to her office in Berlin.
Italian police are investigating after a package ignited at Bologna airport.
The pilot of a private courier plane was told to divert to Bologna after it emerged that the package was addressed to Prime Minister Silvio Berlusconi.
At least 11 similar packages have been found in Athens, mostly addressed to embassies although one was meant for French President Nicolas Sarkozy.
Greek Foreign Minister Dimitris Droutsas told the BBC that, as far as the government could say, the suspension of the airmail service would be limited to 48 hours.
Police carried out controlled explosions on two parcel bombs at Athens airport's cargo terminal on Tuesday evening.
The parcels were addressed to international police organisation Europol in the Netherlands and the European Court of Justice in Luxembourg.
Prime Minister George Papandreou said the authorities were determined to protect Greek citizens, adding that "democracy will not allow itself to be terrorised".
"These irresponsible and cowardly acts will not succeed in hampering our enormous efforts... to re-establish our credibility and revive the economy."
Police are now searching for five men suspected of being members of left-wing group Conspiracy of the Cells of Fire.
They released photographs of all five suspects, after one of two men detained on Monday was said to be part of the group.
Italian police have stepped up security at Rome's main airports, Ciampino and Fiumicino.
Secret services are reported to be investigating a possible link between extreme leftist Greek and Italian groups.
Interior Minister Roberto Maroni told the Ansa news agency that such links did exist and said it was "possible" that Italians might be involved.
Security concerns about cargo flights have already been raised by bomb plots emerging from Yemen.
The European Commission and the EU's Belgian presidency have called a meeting of explosives experts on Friday to examine whether security can be further improved in light of developments in Greece and Yemen.
Angela Merkel said in a newspaper interview on Wednesday that cargo checks in Europe and elsewhere had to be better co-ordinated.
The Greek anarchist group linked to the series of suspect packages, the Conspiracy of the Cells of Fire, aims to spark revolution in Greece during a period of severe austerity.
Greece will hold local elections this weekend, seen as a referendum on the socialist government's handling of the economic crisis.
When Greece was bailed out by the EU and the IMF during its credit crisis in April, Germany insisted on deep budget cuts before the bailout could go through.
Foreign Minister Dimitris Droutsas dismissed the Conspiracy of the Cells of Fire as a "small group of extremists at the edge of Greek society", adding that it had no link at all with international terrorism.