By Renee Maltezou

ATHENS (Reuters) - Thousands of Greek workers were expected to protest austerity cuts as part of annual May Day rallies on Tuesday, days before a national election that risks derailing an international bailout keeping Greece afloat.

Greece's two major private and public sector unions GSEE and ADEDY plan to hold a rally in Athens to mark the national holiday, while the Communist-affiliated PAME group was also scheduled to hold a separate rally.

Police prepared for the violence that has come to mark many such rallies once demonstrators reach the main square in front of parliament, though Athens has not seen major clashes since an unpopular austerity bill was approved in February.

Athens buses, trains and the subway came to a standstill as transport workers staged a 24-hour strike, while Greek seamen held a four-hour stoppage. Public sector offices were shut and hospitals worked on emergency staff.

The rallies and strike come against a backdrop of growing anger and frustration among Greeks over repeated rounds of austerity that have slashed wages and pensions and deepened a recession that is now in its fifth year.

Private sector wages shrunk by a quarter last year alone, while unemployment has soared to a record 21 percent. One in two Greek youth are out of work.

The gloomy national mood has ensured campaigning for Sunday's election has got off to a surly start.

The country's two biggest parties, the Socialist PASOK and the conservative New Democracy, are expected to barely eke out enough support to renew their pro-bailout coalition, which analysts see as the only viable option for Greece to carry out reforms needed for continued aid and staying in the euro zone.

Much of the support the two parties, who have ruled Greece for decades, once enjoyed has now shifted to an array of smaller anti-bailout parties riding high on voter discontent over the austerity measures.

An inconclusive election result could thwart the austerity and reform policies Athens has agreed in exchange for the 130-billion euro bailout that saved the country from bankruptcy.

Greece's lenders have warned that if the country fails to stick to the reforms pledged in return for aid, the country might be forced to abandon the euro.

Most Greeks want to keep the single currency, despite opposing the austerity measures they have been forced to endure since the country's first EU/IMF bailout in 2010.

(Writing by Deepa Babington; editing by Anna Willard)