Rioters hurled petrol bombs at police who responded with tear gas as an anti-austerity demonstration outside parliament turned violent Wednesday, while Greek lawmakers 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 storefronts and set at least one vehicle alight. The clashes were 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 detained.

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 fueled anger among the governing left-wing Syriza party and led to a revolt by many party members against Prime Minister Alexis Tsipras, who has insisted the deal forged early Monday after a marathon weekend eurozone summit was the best he could do to prevent Greece from crashing out of Europe's joint currency....