Demonstrators on Wednesday threw stones and gasoline bombs at police outside parliament during a two-day general strike that unions described as the largest in years.
The protest, which has grounded flights, disrupted public transport and shut down shops to schools in Greece, comes ahead of a parliamentary vote on a fresh package of tax increases and spending cuts required by international creditors in return for crucial bailout cash.
Without the money from its partners that use the euro and the International Monetary Fund, Greece has said it will run out of money within a month.
Most of the 70,000 or so protesters that have converged in central Athens have marched peacefully, but chaos unfolded outside the parliamentary building as crowds clashed with police who tried to disperse them with tear gas.
Some people set fire to a presidential sentry post.
Nearby, groups of protesters tore chunks of marble off building fronts with hammers and crowbars and smashed windows and bank signs.
In the city of Thessaloniki, protesters smashed the facades of about 10 shops that defied the strike and remained open, as well as five banks and cash machines. Police fired tear gas and threw stun grenades.
All sectors, from dentists, state hospital doctors and lawyers to shop owners, tax office workers, pharmacists, teachers and dock workers walked off the job ahead of a Parliamentary vote Thursday on new austerity measures which include new taxes and the suspension of tens of thousands of civil servants.
Flights were grounded in the morning but some resumed at noon after air traffic controllers scaled back their initial strike plan from 48 hours to 12. Dozens of domestic and international flights were still canceled throughout the day.
Ferries remained tied up in port, while public transport workers staged work stoppages but were to keep buses, trolleys and the Athens metro running for most of the day.
In Parliament, Finance Minister Evangelos Venizelos told lawmakers that Greeks had no choice but to accept the hardship.
"We have to explain to all these indignant people who see their lives changing that what the country is experiencing is not the worst stage of the crisis," he said. "It is an anguished and necessary effort to avoid the ultimate, deepest and harshest level of the crisis. The difference between a difficult situation and a catastrophe is immense."
About 3,000 police deployed in central Athens, shutting down two metro stations near Parliament as protest marches began. Police estimated the crowd at least 70,000.
Protesters converged on the square in front of Parliament, banging drums, chanting slogans against the government and Greece's international creditors who have pressured the country to push through rounds of tax hikes and spending cuts.
At least 15,000 demonstrators also gathered in Thessaloniki, Greece's second-largest city.
"We just can't take it any more. There is desperation, anger and bitterness," said Nikos Anastasopoulos, head of a workers' union for an Athens municipality.
Other municipal workers said they had no option but to take to the streets.
"We can't make ends meet for our families," said protester Eleni Voulieri. "We've lost our salaries, we've lost everything and we're in danger of losing our jobs."
Demonstrations during a similar 48-hour strike in June left the center of Athens convulsed by violence as rioters clashed with police on both days while deputies voted on another austerity package inside Parliament.
"We expect that the strike could be the largest" in decades, said Ilias Vrettakos, deputy president of the civil servants' union ADEDY.
"The fact that other sections of society that are suffering from government policies are also participating gives a new dimension to the social resistance by workers and the people in general, and we hope that this mobilization will have an impact on political developments."
Piles of garbage festered on street corners despite a civil mobilization order issued Tuesday to order garbage crews back to work after a 17-day strike.
Earlier in the week, private crews were contracted to remove trash from along the planned demonstration routes, but mounds remained on side streets, along some of the march routes and in city neighborhoods.
Protesting civil servants have also staged rounds of sit-ins at government buildings, with some, including the Finance Ministry, being under occupation for days. Prime Minister George Papandreou appealed on Tuesday for the protests to end.
"I would like to ask all those who occupy ministry buildings, choke the streets with garbage, close off ports, close off the Acropolis, if this helps us stand on our feet again — of course it does not," Papandreou told parliament.
Most stores in the city center, including bakeries and many of the ubiquitous kiosks which sell everything from newspapers, cigarettes and chewing gum to tourist trinkets and snacks, were shut Wednesday.
Several shop owners said they had received threats that their stores would be smashed if they attempted to open during the first day of the strike.
The measures to be voted on in Parliament Thursday come after more than a year and a half of repeated spending cuts and tax increases, and include tax hikes, further pension and salary cuts, the suspension on reduced pay of 30,000 public servants out of a total of more than 750,000, and the suspension of collective labor contracts.
A communist party-backed union has vowed to encircle Parliament Thursday in an attempt to prevent deputies from entering the building for the vote. The reforms have been so unpopular that even some lawmakers from the governing Socialists have indicated they might vote against at least some of them.
But Greece must pass the bill if it is to continue receiving funds from its euro110 billion international bailout. Unless it receives the now long overdue disbursement of an euro8 billion installment, it has said it will run out of funds to pay salaries and pensions by mid-November.
Meanwhile, European countries are trying to work out a broad solution to the continent's deepening debt crisis, ahead of a weekend summit in Brussels. It became clear earlier this year that the initial bailout for Greece was not working as well as had been hoped, and European leaders agreed on a second, euro109 billion bailout.
But key details of that rescue fund, including the participation of the private sector, remain to be worked out.