A bomb has exploded outside a maximum security prison near the Greek capital, Athens, police say.
The blast in Korydallos followed a telephone warning to an Athens newspaper, officials said.
One report says a woman has been injured in the blast, but this has not been confirmed.
There was no immediate claim of responsibility, but correspondents say suspicion will fall on left-wing Greek militant groups.
Athens has seen recent violent protests as the government unveiled tough austerity measures in return for a huge rescue package for its debt-ridden economy.
The BBC's Malcolm Brabant in Athens says that if it turns out to be the work of left-wing guerrillas, it would be a sign that they intend to keep up their violent campaign against the government's austerity programme.
Greek media reported that the device had been hidden in a travel bag.
The powerful blast shattered windows of nearby homes and was heard in the centre of Athens several kilometres away.
Police launched an investigation and sealed off a street next to the prison.
Athens has seen a number of bomb attacks blamed on leftist militants.
In March a bomb outside a public building in the Patissia area of Athens killed a 15-year-old boy.
Earlier attacks targeted banks and government buildings and were attributed to far-left or anarchist groups.
This is the first I recall reading of the 15 yr old boy killed prior to the 3 bank workers.
