Protestors clashed with police, who fired tear gas early Wednesday, several hours before the Greek parliament was to vote on an austerity package.
The vote on a 28.4 billion-euro ($40.8 billion) package of taxes, spending cuts and sell-offs is crucial for Greece to secure international aid to stave off bankruptcy, which is threatening the stability of the eurozone.
The vote is eagerly awaited by other debt-ridden eurozone countries fearing dangerous knock-on effects.
The new measures were drawn up to meet European Union and International Monetary Fund conditions for further bailout support, after a 110-billion-euro rescue last year.
If passed, these creditors could -- as early as Sunday -- authorise 12 billion euros in blocked funds needed to stave off bankruptcy for the Athens government when massive repayments fall due in mid-July.
On Tuesday, police fired tear gas on protestors on the first day of a 48-hour general strike against the belt-tightening package.