Clashes broke out between police and protesters who could be seen throwing rocks and lighting fires in Athens.
Hooded youths ripped up paving stones and set trash bins on fire in central Athens as police gave chase and fired tear gas and stun grenades. Earlier, about 20,000 people had marched peacefully in two separate demonstrations, while another 7,000 protested in the northern city of Thessaloniki without incident.
Workers across the country walked off the job as part of a 48-hour strike while lawmakers debate new reforms.
A vote is scheduled Wednesday on a tough five-year package of tax increases and spending cuts.
The reforms must be passed for the country to get crucial bailout funds from the EU and the IMF to stave off a possible default in July.
If the votes don't pass, Greece could become the first eurozone nation to default on its debts, sending shock waves through the global economy.
The Associated Press contributed to this report.