A car bomb has gone off outside a Bank of Greece building in central Athens, smashing windows in nearby shops but causing no injuries, police and witnesses say.

The blast on Wednesday 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.

A police officer, spealing on condition of anonymity, told Reuters news agency that someone had anonymously warned a newspaper of the attack about 45 minutes before the explosion just before 6am local time.

The caller said the device contained about 70kg of explosives.

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 financial bailouts by the European Union and International Monetary Fund in 2010.

Small-scale attacks against politicians, journalists and businesspeople are frequent in Greece.