The blast occurred at about 0600 (0400 GMT), near a post office in Syntagma Square, close to the Greek finance ministry building.

Nearby cars, cafes and shops were damaged by the explosion.

A Greek daily newspaper received a bomb warning 30 minutes before the blast, state television said, but no one has claimed responsibility for the device.

Police said the bomb appears to have been left on a motorcycle near the finance ministry building.

"It was a very strong explosion, I was thrown back by the sound wave," a delivery driver told state television channel Net. "Luckily the police were on the scene immediately."

The finance ministry is often the target of any demonstrations against Greek government policy.

Strikes planned

The blast coincides with the beginning of a week of heavy strike action in Greece, over changes to public sector contracts.

Greece's largest union has called for transport strikes between 1100 and 1600 (0900 GMT-1400 GMT) on Monday and again on Thursday.

There is also a 24-hour strike planned for Wednesday across the public sector.

Athens has been attacked by bombers before.

The 17 November radical leftist guerrilla group was blamed for a number of attacks and assassinations, but was apparently disabled by a government crackdown, which resulted in 19 of its members being jailed in December 2003.

Other local anarchist groups have mounted attacks in the past, but Athens has been quiet since police action was stepped up before the city hosted the Olympics last year. 

--BBC NEWS