Rioters hurled petrol bombs at police who responded with tear gas as an anti-austerity demonstration outside the Greek parliament turned violent.
The clashes came as Greek politicians began debating contentious measures needed to start negotiations on a new bailout and avoid financial collapse.
Groups of youths among the more than 12,000 protesters smashed shopfronts and set at least one vehicle alight.
It was the first significant protest violence since the left-wing Syriza government came to power in January promising to repeal bailout austerity. Police said at least 50 people were arrested.
The protest was timed to coincide with the start of debate on the Bill, which includes consumer tax increases and pension reforms that will condemn Greeks to years of more economic hardship.
The Bill has fuelled anger among the governing party and led to a revolt by many party members against Prime Minister Alexis Tsipras, who has insisted the deal forged early on Monday after a marathon weekend eurozone summit was the best he could do to prevent Greece from crashing out of Europe's joint currency.
"I must tell you that Monday morning at 9.30, it was the most difficult day of my life. It was a decision that will weigh on me for the rest of my life," said Finance Minister Euclid Tsakalotos.
"I don't know if we did the right thing. But I know we did something with the sense that we had no choice. Nothing was certain and nothing is" he said as the debate kicked off.
Civil servants protested with a 24-hour strike that disrupted public transport and shut down state-run services across the country.
Large numbers of Syriza politicians are almost certain to vote against the package, though the Bill is expected to pass with support from pro-European opposition parties.