Police have been firing teargas in an effort to disperse the crowd
Greek police have fired teargas at protesters outside parliament as MPs prepare to debate new austerity measures required for the EU and IMF bail-out package.
Demonstrators around Syntagma Square in Athens responded by throwing yoghurt and stones.
Thousands are taking part in a general strike, the third in Greece this year.
Ports, public transport and banks are badly disrupted as the main public- and private-sector unions go out on strike.
Prime Minister George Papandreou is seeking support for a new austerity programme of 28bn euros (£24.6bn; $40.5bn) in cuts to take effect from 2012 to 2015.
State-run companies have also joined the walkout, while hospitals are only offering emergency care. However, airports are operating normally after air traffic controllers called off their strike.