Greek police have intercepted a booby-trapped parcel addressed to French President Nicolas Sarkozy, one of four packages to be sent.
A parcel destined for the Mexican embassy in Athens exploded at a courier company, slightly injuring a female employee.
Two more were addressed to the Belgian and Dutch embassies. Police carried out controlled explosions to make them safe.
One official said the devices were “too small to kill”. They are being examined by anti-terrorist police.
At least two suspects have been arrested: they are said to be in their early twenties and to have been carrying weapons.
Earlier reports spoke of two women also being held.
Police said they do not think there was a link with al Qaeda. Instead an extreme Greek leftist group is suspected.
A spokesman described the parcel addressed to Sarkozy as “ridiculous” as it could never have reached its target.
Several homemade bombs have been sent over the past couple of years, since the police killing of a teenager in Athens sparked the country’s worst riots in decades.