A roadside bomb exploded on a main Athens road early today as two police riot squad buses passed but caused no injuries.
The device was placed on Petrou Ralli avenue as the buses headed for the top security Korydallos prison and was detonated from a nearby park, police said.
Athens police chief Mr Vassilis Tsiatouras said the device was "rather small", and that there was no claim of responsibility.
Police officers and bomb experts searched the park for clues and sifting through the remains of the bomb, the first in over a decade to target buses carrying police personnel.
Several radical leftist groups, including the once deadly November 17 group, regularly attacked such buses in the 1980s and 1990s.
Members of the group who were caught two years ago are being held at Korydallos prison.