Protests have spread in central Athens, amid anger over austerity measures being debated by parliament.
Protesters threw stones and petrol bombs, and police fired tear gas. A number of people were injured and at least 10 buildings were set on fire.
PM Lucas Papademos urged calm, saying violence had no place in a democracy.
MPs are voting on a bill to introduce the measures, which are being demanded in return for a 130bn-euro ($170bn; £110bn) bailout to avoid default.
The BBC's Mark Lowen in Athens says the violence is the worst the city has seen for many months.
Protesters hurled flares and chunks of marble torn up from the square. Some tried to break through a cordon of riot police around the parliament.
Several historic buildings, including cafes and cinemas, were in flames.
Syntagma Square - in the heart of Athens - is cloaked in a hail of tear gas, our correspondent adds.
Ioannis Simantiras, 34, said the protesters were boxed in by the police.
"Nobody could get away from the gas," he told the BBC.
"When it engulfed everybody, and everybody was choking the police drew back and opened up a corridor for us away from the parliament - that's when everybody made a run for it."
Mr Papademos said Greece did not have the luxury of such protests in such difficult times.
"Vandalisms, violence and destruction have no place in a democratic country and won't be tolerated," he said in a speech in parliament before the vote.
Running battles with police are still continuing in parts of the capital.
Earlier, tens of thousands of people voiced their anger over the proposed bill in what was the second consecutive day of mass protests.
Some reports say as many as 80,000 people joined the demonstration in Athens, with another 20,000 protesting in Thessaloniki.
A number of lawmakers have threatened to vote against the bill, but analysts say it should still have enough support in parliament to pass.
Pasok, the largest party, and its coalition ally New Democracy - which have both backed the bill - account for more than 230 deputies out of a total of 300.
Lawmakers are also due to vote on a deal with private banks which could see 100bn euros of Greek debt written off.
If the measures are not approved, other eurozone nations and the International Monetary Fund (IMF) say Greece will get no money from them and will face bankruptcy in March.
Athens has to repay nearly 14.5bn euros in maturing debt on 20 March.
'Ground Zero' warning
The debate began in the early afternoon on Sunday, but the vote did not begin until after midnight (22:00 GMT).
In a TV address on Saturday, Mr Papademos warned that Greece was "just a breath away from Ground Zero".
"The social cost of this programme is limited in comparison with the economic and social catastrophe that would follow if we didn't adopt it," he said.
Savings would be lost, the government would be unable to pay wages or salaries, and imports of fuel, medicine and machinery would be disrupted, he added.
The austerity measures include:
- 15,000 public-sector job cuts
- liberalisation of labour laws
- lowering the minimum wage by 20% from 751 euros a month to 600 euros
- negotiating a debt write-off with banks.
They were presented to eurozone ministers in Brussels on Thursday evening.
The Greek cabinet has approved the measures but five government ministers resigned.
The eurozone bloc wants a further 325m euros in savings for this year and also insists that Greek leaders give "strong political assurances" on the implementation of the packages.
Greece cannot service its huge debt, and there are fears that a default could endanger Europe's financial stability and even lead to a break-up of the eurozone.
But many Greeks feel they are already squeezed almost to breaking point and cannot take any more cuts, our correspondent says.
Some are even saying Greece should leave the eurozone to be able to devalue its former currency, the drachma, and ease the debt stranglehold.