No injuries or arrests were immediately reported from the clashes, which involved police responding with tear gas after being targeted by rocks and Molotov cocktails hurled by protesters.
Greek public- and private-sector unions launched a 48-hour nationwide strike on February 10 to protest new austerity measures that the government negotiated this week with international lenders.
The labor action is aimed at shutting down public transportation, schools, ferries, courts, museums, and government offices. Hospitals are working with emergency staffing.
The unions oppose the government's latest austerity package, which includes a 22 percent public-sector wage reduction, layoffs, privatizations, and new taxes.
European Union ministers, however, said on February 9 that the new package was still not sufficient for international lenders to approve a 130 billion-euro ($170 billion) bailout for the troubled Greek economy.
German Finance Minister Wolfgang Schaeuble said the Greek plan was "not at a stage where it can be signed off."
Eurozone finance ministers on February 9 suspended a decision on approving the bailout package for Greece and set conditions for Athens to meet by February 15.
At a meeting just hours after Greek political leaders announced they had agreed on reforms required to secure the bailout from international lenders, Luxembourg’s Prime Minister Jean-Claude Juncker said Greece needed to meet three more conditions.
Juncker said those conditions were: identifying an extra 325 million euros in savings for 2012; the Greek parliament's passage of the full package of cuts and reforms on February 12; and "strong political assurances" from Greek party leaders of support for promised reforms after elections that could come as early as April.
The European Union's economic affairs commissioner, Ollie Rehn, said the eurozone was "seriously considering" forcing Greece to set up a separate fiscal account dedicated to repaying its debt.
Compiled from agency reports