Police in Greece says a bomb has exploded outside a top security prison near Athens. A telephone warning had been given. No group has said it was responsible but similar attacks in the past had been blamed on left wing guerillas. <br/>This was a big explosion. The blast was heard in central Athens more than eight kilometers away. There are unconfirmed reports that a woman has been hurt. According to a police spokesman, a bomb was placed next to the wall housing women prisoners at the top security jail. <br/>Earlier, tourism officials had said that at least twenty thousand people cancelled hotel reservations in Athens following the violent protests last week. <br/>