Greek police have fired teargas at demonstrators in central Athens at the start of a 48-hour strike against austerity measures.
With Greece facing the prospect of bankruptcy, parliament is due to vote this week on a package of spending cuts, tax increases and privatisations agreed as part of a massive bailout aimed at averting the eurozone's first default.
In a boost to embattled Prime Minister George Papandreou, one of three deputies from his ruling Socialist party who had suggested they would oppose the bill has said he had decided instead to support it.
'I have made the decision to vote for the plan because national interests are more important than our own dignity,' Thomas Robopoulos told Reuters.
Labour unions launched a 48-hour strike to protest the measures.
Transport and public services have been badly hit, schools are shut and many shops and businesses are also closed. Aside from the demonstrations, the streets of central Athens are virtually deserted.
Most attention is focused on Syntagma Square in front of the parliament and more than 5,000 police were deployed to the centre of the capital to deal with the protests.
The rally was initially peaceful but by early afternoon hundreds of youths, many wearing gasmasks and scarves covering their faces, hurled masonry chipped off buildings in the square at riot police who responded with teargas.
18 people were detained and five were formally placed under arrest, police said.
After night fell, thousands of peaceful demonstrators returned to Syntagma to chant insults at parliament, as they have done every night for over a month.
Deep in deficit and unable to borrow on financial markets, Greece depends on international support to keep going. A default would spread contagion around the 17-nation single currency area and cause a deep shock to the global economy.
Initial vote due tomorrow
MPs are due to vote on the framework of the package tomorrow and on its implementation on Thursday.
Prime Minister George Papandreou's socialists have a majority, with 155 deputies in the 300-seat house, but some MPs have warned they may vote against the austerity plan.
Analysts say the risk of the mid-term plan not passing is small, especially since it may win support from smaller conservative groups.
If either or both of the parliamentary votes are rejected, then Greece theoretically faces bankruptcy in a matter of days.
However, eurozone sources have said they are working on a contingency plan to provide at least emergency liquidity to keep the country going.
EU Commissioner for Economic and Monetary Affairs Olli Rehn has urged the Greek parliament to adopt the austerity programme, saying it was the 'only way to avoid immediate default'.
'This week Greece faces a critical juncture. Both the future of the country and financial stability in Europe are at stake,' Mr Rehn said in a statement.
'I trust that the Greek political leaders are fully aware of the responsibility that lies on their shoulders to avoid default.
'The only way to avoid immediate default is for Parliament to endorse the revised economic programme.'
