"The explosive device went off around 0230 GMT, shattering glass at nearby buildings. There was a warning call to police a few minutes earlier but no claim of responsibility," a police official said on condition of anonymity.

Greece's anti-terrorist squad investigating the blast found pieces of a table clock and believes the explosive material used was common dynamite but more will be known after lab tests, the police official said.

There has been a string of bombing attacks against domestic and foreign businesses in Greece since widespread rioting in December last year, sparked by the police killing of a teenager.

In a separate incident later on Friday, attackers threw gas canisters at the entrance of the Institute of Immigration Policy in central Athens, causing damages but no injuries, police said.