The leader of the left-wing Syriza party Alexis Tsipras has said Greece is "leaving behind disastrous austerity", after his party claimed victory in the country's general election.
And the 40-year-old told jubilant supporters the country's group of lenders "is finished".
He was speaking after the Greek Prime Minister Antonis Samaras, who heads the conservative New Democracy party, conceded defeat to Mr Tsipras.
Partial election results suggest Syriza has secured 36.5% of the vote, compared to 27.7% for the New Democracy party.
This indicates the party would get 150 seats in the 300-seat parliament - one short of an overall majority.
Hundreds of flag-waving supporters have turned out in central Athens to celebrate.
But the victory for Syriza raises the prospect of a stand-off with European creditors, with Mr Tsipras promising to renegotiate the repayment terms of Greece's debt.
The Greek government was forced to undertake deep budget cuts and fiscal reforms as a condition for a €240bn euro bailout in 2010 from the so-called "Troika" - the group of creditors made up of the European Union, the International Monetary Fund and European Central Bank (ECB).
Many in Greece feel slashed public spending has hit the most vulnerable hardest, while leaving the tax evasion and corruption of the apparent elites untouched.
In his victory speech, Mr Tsipras said: "The sovereign Greek people today have given a clear, strong, indisputable mandate.
"Greece leaves behinds catastrophic austerity, it leaves behind fear and authoritarianism, it leaves behind five years of humiliation and anguish.
"The verdict of our people means the Troika is finished.
"The new Greek government will be ready to cooperate and negotiate for the first time with our peers a just, mutually beneficial and viable solution."
Mr Syriza's refusal to continue meeting the austerity demands placed on Greece by its creditors has sparked fears the country may be unable to repay its debts, which could force the country's exit from the Eurozone.
Germany's Bundesbank has warned Greece needs to reform to tackle its economic problems, while the euro fell nearly half a US cent.
Mr Samaras had insisted voters would be making a huge mistake to elect Syriza at a time when painful fiscal reforms may be about to pay off.
After conceding defeat, he told supporters: "My conscience is clear because I told the truth to the Greek people until the very end.
"I received a country that was almost destroyed and I was asked to hold a hot potato and I did that."