A bomb blast at the offices of Greece's public order ministry in Athens has killed a close aide to the minister responsible for counter-terrorism.
Police said the victim had opened a parcel bomb.
The explosion happened only metres away from the office of the minister, Michalis Chryssohoidis, who was unhurt.
Greek Prime Minister George Papandreou said the bombing was a terrorist attack. So far no group has said it was behind the bomb.
The blast was so powerful that some in the heavily guarded building thought it had been struck by an earthquake, the BBC's Malcolm Brabant in Athens reports.
The victim was later identified as Giorgos Vassilakis, a 50-year-old father of two.
Visibly shaken, Mr Chryssohoidis said he had "lost a valuable and beloved colleague".
"We cannot be scared and we cannot be terrorised. These cowardly murderers will be brought to justice."
The minister added that the parcel had been meant for him.
Mr Papandreou also branded the bombers "cowards", adding: "They will get the response that they deserve not only from the state but also from all of society. The terrorists will not reach their objective."
Greek terrorism expert Dr Athanasios Drougas told the BBC that the bomb was probably the work of Revolutionary Struggle, the country's most deadly active guerrilla group.
In recent months police have made major breakthroughs against Revolutionary Struggle and another militant organisation, Conspiracy of Fire.
Dr Drougas said Revolutionary Struggle was sending a message that its was not defeated and was still capable of striking at the heart of the Greek government.