The strike is taking place as European leaders are in Brussels for a summit in which Greece’s economic fate is likely to feature large.
Greece is currently preparing a 13.5bn-euro (£11bn; $17.7bn) austerity package to satisfy the “troika” of International Monetary Fund, European Commission and European Central Bank lenders in return for its next 31.5bn-euro tranche of aid.
The country is due to run out of money next month.
However, trade union leaders says they hope to show EU leaders that a new wave of wage and pension cuts will only worsen the plight of the Greek people.
Greece is in its fifth consecutive year of recession and more than a quarter of its workforce is unemployed.
“Just once, the government ought to reject the troika’s absurd demands,” said Yannis Panagopoulos, head of the GSEE private sector union.
“Agreeing to catastrophic measures means driving society to despair, and the consequences as well as the protests will then be indefinite,” he added.
The BBC’s Mark Lowen in Athens says cracks are already appearing within the coalition government as party leaders disagree over job losses and other austerity measures.
Anger has led to a loss of faith in the state, he says, with Greeks increasingly turning to political extremes such as the neo-Nazi Golden Dawn party.
Although the Greek government has vowed to stay the course, social unrest could yet prove explosive, our correspondent adds.