A massive bomb has gone off outside Athens' main prison, damaging dozens of shops and homes up to four blocks away, police said. A police official said the device was placed inside a rubbish container close to the wall of Korydallos prison, where the convicted members of the November 17 guerrilla group are serving sentences. A man and a woman were slightly injured by broken glass. The blast was heard miles away, according to witnesses. A police official said: "It was a very powerful bomb, probably the biggest we have had in years, but we had evacuated the area." An unidentified caller had warned a Greek TV station and a newspaper that a bomb would blow up outside the prison in minutes. Police rushed to evacuate the area before the device went off on Thursday evening. The official added: "We have counted 55 shops and homes damaged so far in a radius of several blocks." Most buildings had their windows blown out.