Greeks have begun voting in an early general election crucial for the country's financial future, with the radical left Syriza party of Alexis Tsipras tipped as the favorite to win, although possibly without enough votes to form a government.

Opinion polls leading up to Sunday's vote all showed Tsipras ahead of conservative Prime Minister Antonis Samaras.

Syriza has run on a campaign of renegotiating the country's 240 billion euro international bailout deal, and has pledged to reverse many of the reforms pushed through to qualify for the rescue loans that have kept Greece financially afloat for the past four years.

The anti-bailout rhetoric has renewed fears of Greece's ability to definitively emerge from a financial crisis that saw a quarter of its economy wiped out and hammered the euro.