Greek unions planned protests in Athens and other cities on Thursday over the prospect of new austerity cuts demanded by the country's international creditors.
A small group of activists from the pro-Communist PAME trade union occupied the finance ministry in the early morning, taking down the EU flag and unfurling a massive banner which said Greeks "had bled enough."
"We have bled enough, we have paid enough," the banner, which portrayed leftist Prime Minister Alexis Tsipras alongside his pro-reform predecessors, socialist George Papandreou and conservative Antonis Samaras, read.
PAME and civil servants' union Adedy were to hold protests in Athens, Thessaloniki and other cities in the evening amid reports that the government were nearing a new loan deal entailing fresh cuts.
Tsipras was to meet with European Commission chief Jean-Claude Juncker in Brussels Thursday after he agreed with the leaders of France and Germany to intensify efforts for a bailout deal aimed at preventing Athens from going bankrupt.
The Greek premier's anti-austerity Syriza party won elections in January with a promise to ease the hardship caused by five years of austerity imposed under two international bailouts since 2010.
Unemployment climbed to 26.6 percent in the first quarter of the year, compared to 26.1 percent in the previous three-month period, the state statistics agency said Thursday.
An opinion poll published late Wednesday showed that 53 percent of Syriza voters favoured breaking off the debt negotiations with the EU and IMF.
However, 50.2 of all respondents in the Marc poll for Alpha TV said they wanted the government to reach a deal with the creditors, and 80.3 percent said a deal was likely.
Greece's creditors have refused to release the last 7.2 billion euros ($8.1 billion) remaining in the country's EU-IMF bailout, which is due to expire on June 30, unless Athens agrees to tougher reforms.
Without the cash Greece will be unable to pay its foreign debts, after already ordering local authorities to hand over their cash reserves to meet earlier commitments.
A default could send Greece crashing out of the euro.