Protesters calling for a general strike have taken over the finance ministry building in Athens just as the Greek government wraps up tough negotiations with international officials on new austerity measures.
About 200 protesters from the communist party-backed PAME union blockaded the entrance to the ministry from dawn on Friday, preventing employees from entering.
They hung a banner over five stories of the front of the building and took down the European flag from the top of the ministry, replacing it with their own union flag.
The protest came as experts from the European Union, European Central Bank and International Monetary Fund (known collectively as the troika) were wrapping up a review of Greece's implementation of economic reforms in return for 110 billion euro (£97.5bn) in rescue loans from the EU and IMF.
The three bodies were to issue a statement on their review later Friday, officials said. The review is crucial towards determining whether Greece will receive a fifth tranche, worth 12 billion euro (£10.6bn) of bailout loans agreed last year.
Greece has so far received 53 billion euro (£47bn) from last May's rescue deal.
Greek officials were also completing tough negotiations on the details of more austerity measures needed to ensure the country can avoid defaulting on its debts.
The original bailout plan envisaged the country being able to tap bond investors next year, but with the interest rates on Greek bonds remaining exceptionally high, that appears increasingly unlikely.
Last month, Finance Minister George Papaconstantinou announced remedial austerity measures worth about 6.4 billion euro (£5.6bn) for this year, in order to meet the target of reducing the deficit to 7.5% of gross domestic product, from 10.5% in 2010.
Greece's woes have been compounded by repeated downgrades of its credit ratings. Moody's warned on Wednesday that the country had a 50-50 chance of defaulting on its debts.