Three people including a pregnant woman have died after protesters threw petrol bombs into a bank in central Athens.
Tens of thousands of Greeks took to the streets of the capital, clashing with police, who responded with repeated rounds of tear gas and flash bombs.
The violence is a blow to Prime Minister George Papandreou's plans to push through tough budget cuts demanded by the European Union and International Monetary Fund in exchange for a 110 billion euro aid package.
Officials said two men and a woman died of asphyxiation in a two-storey commercial building that housed a branch of the Marfin bank.
Police said two women and one man, aged between 30 and 40, were working at the bank on Stadiou Avenue when they choked on smoke after protesters broke the windows and tossed in Molotov cocktails.
Marfin bank officials said one of the female victims was pregnant. Firemen had to restrain a distraught elderly woman outside the bank, who wept and cried "my child, my child".
Greek Prime Minister George Papandreou said: "We are deeply shocked by the unjust death of the three employees, our fellow citizens, who were the victims of a murderous action. To those who are mourning we say that the culprits will be found and brought to justice."
Police put the march at roughly 30,000 people. But eyewitnesses said there were more than 50,000 - easily the biggest protest since Mr Papandreou took office last October and began introducing austerity measures.