In an anonymous call to an Athens newspaper, a man said the far-left Popular Will group carried out Wednesday night's attack but did not elaborate on the reasons.

The blast _ in a highly guarded part of the capital, opposite the private office of Greece's interior minister _ targeted a firm linked in local media reports to deals concerning security systems for the 2004 Athens Olympics.

Several of those deals are under investigation for alleged fraud, including transactions between the Greek state and the German industrial giant Siemens AG.

Police said the bomb contained a small quantity of explosives and was detonated by mobile phone.

Popular Will has claimed responsibility for another three small bomb attacks over the past five years, against a courts complex, a Siemens office and the offices of a Greek think tank. Only one of the bombs exploded, without causing injury or serious damage.

Greek militant groups have increased their attacks over the past few months, following December's police shooting of a teenager that caused the country's worst riots in decades. Nobody has been killed so far, but a riot policeman was severely wounded in a shooting attack earlier this year.