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As lawmakers neared a vote on unpopular budget cuts and labour reforms, the rally attended by about 100,000 disintegrated into violence, with protesters and riot police fighting running battles in Syntagma Square.
More chaos reigned inside the assembly, where the session was briefly interrupted when parliamentary workers went on strike to protest against a clause that would have cut their salaries. In a humiliating about-face, the government was forced to cancel the measure to allow the session to resume.
Outside, loud booms rang out as protesters hurled petrol bombs and rocks at police, who responded with teargas, stun grenades and water cannons - the first time they had been used in an anti-austerity protest. Billowing smoke and small fires could be seen on a street next to parliament.
The violence erupted as a handful of protesters tried to break through a barricade to enter parliament, where Prime Minister Antonis Samaras is expected to scrape a win for the belt-tightening measures despite opposition from within his coalition.
Earlier in the evening, Greeks outside the parliament braved a downpour holding flags and banners saying "It's them or us!" and "End this disaster!"
Protesters - some chanting "Fight! They're drinking our blood" - packed the square and side streets in one of the largest rallies seen in months.
Some held aloft huge Italian, Portuguese and Spanish flags in solidarity with other nations enduring austerity.
"These measures are killing us little by little and lawmakers in there don't give a damn," said Maria Aliferopoulou, a 52-year-old mother of two living on 1,000 euros a month.
"They are rich, they have everything and we have nothing and are fighting for crumbs, for survival."
Public transport was halted, schools, banks and government offices were shut and garbage piled up on streets on the second day of a two-day national strike against the cuts.
Backed by the leftist opposition, unions say the measures will hit the poor and spare the wealthy, while deepening a five-year recession that has wiped out a fifth of the country's output and driven unemployment to a record 25 percent.
The cuts and tax hikes expected to be worth 13.5 billion euros are required to unlock a loan tranche of more than 31 billion euros ($40 billion) from the European Union and International Monetary Fund bailout.
The vote is the biggest test for Samaras's government since it came to power in June. A 'yes' will give Athens cash to shore up its ailing banks and pay off debt due later this month.
ON THE BRINK
EU Economic and Monetary Affairs Commissioner Olli Rehn called on the Greek parliament to do its part in securing its next tranche of bailout aid by passing the measures.
But protesters outside said they were on the brink.
"You live in constant fear and uncertainty. You never know what's waiting for you around the corner," said Panos Goutsis, 58, who works in a small corner shop in Athens.
"How many times will they tell us these are the last measures? We're sick of hearing it."
Greeks have been angered by the relaxed approach consecutive governments have taken towards catching tax cheats, with many saying officials have dragged their feet on investigations to protect a wealthy elite.
Following the publication last month of a list of more than 2,000 wealthy Greeks with Swiss bank accounts, the Swiss government said on Wednesday it was hoping to clinch a swift deal with Athens on taxing secret holdings.
The austerity measures being debated in parliament are accompanied by steps to make it easier for businesses to hire and fire workers.
The junior ruling Democratic Left party has refused to support these, saying they undermine already eroded labour rights. Several MPs from the second ruling party, Socialist PASOK, have also wavered.
But Samaras's New Democracy and the remaining PASOK MPs should be able to push the measures through, with around 155 of parliament's 300 votes.