A bomb blew up outside the Greek central bank in Athens Thursday morning, hours before Greece returns to the international bond market.
Police say the blast shattered windows but injured no one.
Anonymous callers telephoned warnings to a newspaper and a news website about an hour before the blast. No one has claimed responsibility. Telephone warnings traditionally precede such attacks in Athens.
Greece will sell government bonds on the world market Thursday for the first time since 2010. Officials say it will issue a five-year bond.
The market was closed to Greece when its economy collapsed four years ago, forcing it to accept a bailout from the European Union and International Monetary Fund.
The bailout brought Greece back from the edge of bankruptcy, but forced it to make highly unpopular spending cuts and tax hikes.