A Greek state TV exit poll is projecting that anti-bailout party Syriza has won parliamentary elections in an historic first for a radical left-wing party in Greece.
But it is unclear whether Syriza has won a decisive enough victory over Prime Minister Antonis Samaras's incumbent conservatives to govern alone. For that, they need a minimum 151 of parliament's 300 seats.
A senior minister in Greece's governing conservatives conceded defeat to the radical left Syriza party.
"We lost," Health Minister Makis Voridis told Mega TV. "The extent of that result is not yet clear."
Mr Voridis, the conservative party's parliamentary spokesman, said the government's austerity policies, implemented to secure vital international bailouts, "make sense" but were cut short before they could bear fruit.
The centrist Potami (River) party is battling for third place with the Nazi-inspired, extreme right-wing Golden Dawn, whose leadership is in prison pending trial for running a criminal organisation.
Syriza leader Alexis Tsipras has promised to renegotiate the country's 240 billion-euro (£179bn) international bailout deal. He has pledged to reverse many of the reforms that creditors demanded in exchange for keeping Greece financially afloat since 2010.
His anti-bailout rhetoric has renewed doubts over Greece's ability to emerge from its financial crisis that has seen a quarter of its economy wiped out, sent unemployment soaring and undermined the euro, the currency shared by 19 European countries.
Greece's creditors insist the country must abide by previous commitments to continue receiving support, and investors and markets alike have been spooked by the anti-bailout rhetoric.
Greece could face bankruptcy if a solution is not found, although speculation of a "Grexit" - Greece leaving the euro - and a potential collapse of the currency has been far less fraught than during the last general election in 2012.
"What's clear is we have a historic victory that sends a message that does not only concern the Greek people, but all European peoples," Syriza party spokesman Panos Skourletis said on Mega television. "There is great relief among all Europeans. The only question is how big a victory it is."
Mr Skourletis said the election results heralded "a return of social dignity and social justice. A return to democracy. Because, beyond the wild austerity, democracy has suffered."
Greeks have faced years of austerity measures, including cuts in wages, pensions and government spending, and tax increases. Greece's unemployment rate is 25.5%.
The exit poll on state-run Nerit TV projected Syriza as having won with between 35.5 and 39.5% - or 146-158 seats, compared to Mr Samaras's New Democracy with 23-27% - or 65-75 seats.
If Syriza falls shy of the 151 seats necessary to form a government on its own, it will have to seek support from other parties - either in a minority government or as a coalition.
A Syriza government will see Mr Tsipras becoming Greece's youngest prime minister in 150 years.