Riot police fired tear gas to break up protesters who were hurling rocks, sticks and bottles.
The crowds are rebelling against severe budget cuts.
The Greek government is considering raising taxes and cutting funding to meet the demands for a crucial international bailout.
Greece will go broke within weeks unless it passes a new austerity plan.
The tough new package must pass two crucial votes in the Greek parliament on Wednesday and Thursday.
If it fails, Greece will default on the enormous loans it has already taken to keep the country running.
A crippling 48 hour strike has brought the country to a standstill. Bus stops are empty. Train stations are locked up.
Tourists hoping to hop on a ferry to the Greek islands found themselves stranded on the docks.
And security forces are bracing for more violence. Five thousand police have been deployed to protect government buildings in the days ahead. Charlie D'Agata, CBS News.
Greece is already battling a 16 percent unemployment rate.
Demonstrators say new taxes will hit low paid employees the hardest, and the deep budget cuts will take a big chunk out of retirement funds.