The explosion occurred at 0125 GMT outside the Athens appeals court after calls warning of a bomb had been made to the media, police said.
![The suspect pleaded guilty to threatening and offensive behaviour [Task & Purpose]](data:image/svg+xml;charset=utf8,%3Csvg%20xmlns%3D'http%3A%2F%2Fwww.w3.org%2F2000%2Fsvg'%20width%3D'100'%20height%3D'100'%20data-ring-placeholder%3D'1'%3E%3C%2Fsvg%3E "The suspect pleaded guilty to threatening and offensive behaviour [Task & Purpose]")
A police source told AFP that the device was a "powerful timer bomb."
Greek far-left groups have frequently used such bombs in the past.
The caller gave police 40 minutes to clear the area, just a short distance from Athens police headquarters.
The blast broke several windows in the court facade and the building was closed for the day with all court cases suspended.
"This criminal act is heinous and reproachable," Justice Minister Stavros Kontonis said in a statement.
Anti-terror police cordoned off the scene.
According to a court guard, the bomb was inside a bag left by two men who fled on a motorbike, state news agency ANA said.
There was no immediate claim of responsibility.
However, the incident occurred amid a bitter row over foreclosures of properties over longstanding debts to Greek banks.
The government has tabled new legislation, hardening rules against anti-foreclosure protesters.
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