ATHENS (Reuters) - A powerful bomb exploded outside Athens' main prison on Thursday, causing extensive damage, but no injuries, police said.

"It was a really strong explosion that was heard kilometers away," said a police official, who requested anonymity.

The top security Athens prison is in the Korydallos suburb, west of Athens.

Greek media reported the bomb was placed inside a garbage container close to the prison wall.

The official said an unidentified person called a Greek TV station and warned that a bomb would blow up outside the prison.

A 15-year-old boy was killed and his mother and sister wounded after a bomb exploded outside a building in central Athens in March.

Bomb attacks by militant leftist groups are frequent in Greece and usually target police, public buildings or businesses. March's explosion was the first to kill someone in years.

Urban violence increased in the country after the police shooting of a teenager in December 2008.

(Reporting by Dina Kyriakidou; Writing by Angeliki Koutantou; Editing by Jon Hemming)