A bomb has exploded outside the main prison in Athens, Greek police say.
The blast in the Korydallos suburb of the city 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. The extent of damage to the prison is not yet known.
There was no immediate claim of responsibility, but correspondents say suspicion will fall on radical left-wing Greek militant groups.
Athens has seen recent violent protests as the government unveiled tough austerity measures designed to tackle Greece’s huge budget deficit.
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