People ran out of the courthouse in Thessaloniki as an explosion was heard
A bomb has exploded outside a courthouse in Thessaloniki, hours after an earlier blast at a prison near the Greek capital Athens.
Two people were reported to have been injured - one in each of the blasts.
Both bombs came after warning calls to local newspapers. The bomb near Athens caused extensive damage to buildings.
There were no immediate claims of responsibility, but police linked the first explosion to left-wing Greek militant groups.
Several members of the left-wing groups are being held at the maximum-security prison that was targeted in Korydallos, near Athens, on Thursday evening.
In Thessaloniki, Greece's second city, police said most people had been evacuated from the courthouse when the bomb there exploded.
One man was reported to have sustained minor injuries.
Greek media reported that the device that exploded near Athens 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. One woman was injured.
Police launched an investigation and sealed off a street next to the prison.
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.
It has also 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.