Several thousand protesters were marching through central Athens, while another 10,000 were demonstrating in Thessaloniki.
The country has been depending on bailout loans from the International Monetary Fund and other European countries since May 2010. In return, it has implemented a series of strict austerity measures to reform its economy.
They have included deep cuts to state salaries and pensions and repeated rounds of tax hikes, measures which many blame for prolonging a deep recession that is now in its sixth year. Unemployment is above 27 percent, the highest in the European Union, while it reaches nearly 60 percent for those under the age of 25.
"Strikes are temporary, no matter how long they last. The consequences of the government's measures are permanent," said Themis Kotsifakis, president of the teachers' union.
Debt inspectors from the IMF, European Commission and European Central Bank, jointly known as the "troika," are due back in Athens to review progress on reforms next week.
The country's main private sector union was joining the strike with a four-hour work stoppage in the middle of the day, while two demonstrations were planned for central Athens: one by a Communist Party-backed union in the morning and the other by the main civil servants' union at around midday.
High school teachers have also embarked on rolling five-day strikes, while state hospital doctors walked off the job for three days from Tuesday.
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Raphael Kominis in Athens contributed.