Greek labor unions launched a 48-hour general strike Wednesday ahead of a crucial parliamentary vote over fresh austerity measures.

The strike was organized by ADEDY and GSEE, the two main umbrella unions of public and private sector employees that represent over 2.5 million workers. The unions consider it one of the biggest mobilizations since the debt crisis started two years ago.

The strike so far has shut down public sector organizations, schools, courts and hospitals that run on skeleton personnel. Public transport services have been crippled, including airports and ports, as strikers gather in two main squares in central Athens to march towards the parliament.

The strike came ahead of a parliamentary vote Thursday over a new austerity bill designed to escape a default that could trigger a domino effect across the eurozone and an European Union summit in Brussels this weekend to finalise the second bailout package to Greece.

Prime Minister George Papandreou failed to win the support of Antonis Samaras, the New Democracy party leader and main opposition conservative, for a "common front" during their latest meeting on Tuesday evening.

As Greece still struggles to avoid a financial collapse, painful austerity drive and delays in critical structural reforms, the socialist government may face a "revolt" within party ranks, Greek media reported.

Former Labor Minister Louka Katseli has said that she would vote against the new austerity measures that include further pay cuts, tax increases and a plan to reduce state spending by forcing 30,000 civil servants to early retirement by the end of the year.

Another deputy stepped down from his post on Monday, arguing that the policies lay "unbearable" burden on the low and middle-income households.

Thousands of anti-riot police were stationed across the capital amidst fears for violence, since similar mass demonstrations over the past 18 months ended in violent clashes between police and hooded anarchists.

Three employees died when a bank branch near the parliament was torched by rioters in May 2010, as the debt-ridden country was granted the first rescue package from the European Union/International Monetary Fund creditors.

Athens witnessed similar scenes of chaos during a rally on a general strike day in June this year, as the parliament approved another set of austerity policies to secure further vital bailout loans by lenders.

Papandreou's ruling PASOK party holds a 154-seat majority in the 300-member parliament. Local analysts expect that the draft bill will be passed, so that Greece will secure the sixth installment of the aid package to avert default in November, but the implementation of the measures will not be easy.