According to local media, the occupations began in the early hours of Thursday and will continue until Friday.
The protesters are opposing a new round of pay-cuts and layoffs imposed by the government and aimed at slashing its runaway deficit.
The protesters say they want to prevent the Greek Finance Minister from meeting the EU, the IMF, and the European Central Bank officials in Athens.
Greece was saved from bankruptcy by international creditors last year. Economists believe more financial bailouts to Greece could plunge the euro-zone into an even deeper crisis.
A troika of European Union, European Central Bank and International Monetary Fund inspectors returned to Greece after quitting the country unexpectedly on September 2, accusing Athens of not taking enough measures to deserve a bailout.
Without new funds, Greece could run out of cash to pay state wage and pension expenses as soon as next month. A Greek default on debt repayments could wreck the balance sheets of banks across Europe and unleash a crisis in the global financial system.