A car bomb went off outside a Bank of Greece building in central Athens early this morning.
Windows in nearby shops were smashed but there were no injuries, police said.
The blast struck hours before Greece planned its first foray into the international bond markets since it plunged into a debt crisis four years ago, and a day before a visit by German Chancellor Angela Merkel.
An anonymous caller warned a newspaper of the attack about 45 minutes before the explosion just before 6am (4am Irish time).
The caller said the bomb contained about 70kg of explosives, according to a police official, who spoke on condition of anonymity.
There was no immediate claim of responsibility, but police believed leftist or anarchist groups were behind it, the official said.
Witnesses saw debris strewn across the street in a busy part of the capital lined with banks and shops.
Makeshift bomb and arson attacks have escalated since Greece adopted unpopular austerity measures in exchange for multi-billion euro bailouts by the European Union and International Monetary Fund in 2010.
Small scale attacks against politicians, journalists and business people are also frequent in Greece.