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 6 a.m. (0300 GMT), saying it contained about 70 kg (155 pounds) of explosives, said 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.
Witnessess saw debris strewn across the street in a busy part of the capital lined with banks, shops and a mall.
Makeshift bomb and arson attacks have escalated since Greece adopted unpopular austerity measures in exchange for multibillion euro bailouts by the European Union and International Monetary Fund in 2010.
Small scale attacks against politicians, journalists and businesspeople are frequent in Greece, with its long history of political violence.