Anti-austerity Syriza party has won Greece's general election, putting the country on a possible collision course with the EU over its massive bailout.
With nearly 75% of the votes counted, Syriza is projected to win 149 seats, just two short of an absolute majority, though that number could change.
Left-wing party leader Alexis Tsipras, who wants to renegotiate Greece's debt, said "the Greeks wrote history".
The governing centre-right New Democracy has come a distant second.
Outgoing Prime Minister Antonis Samaras has admitted defeat and phoned Mr Tsipras to congratulate him.
Syriza leader Alexis Tsipras: "Today the people of Greece wrote history"Syriza's result will send shockwaves through Europe, BBC reports from Athens.
A majority of voters in Greece have essentially rejected a core policy for dealing with the eurozone crisis as devised by Brussels and Berlin, our correspondent adds.
In Germany, Bundesbank President Jens Weidmann said he hoped "the new Greek government will not make promises it cannot keep and the country cannot afford".
The election result is expected to be one of the main issues during Monday's meeting of 19 eurozone finance ministers.
Belgium's representative Johan Van Overtveld was quoted by VRT network as saying that Greece "must respect the rules of monetary union", although he added that there was room for some - but not much - flexibility.
British Prime Minister David Cameron - whose country is not a member of the eurozone - said the result of the Greek election would "increase economic uncertainty across Europe".
Meanwhile, the euro fell to $1.1098 against the dollar - the lowest level in more than 11 years.
'Thing of the past'
Addressing his jubilant supporters in front of Athens' university, Mr Tsipras said Greek voters gave Syriza "a clear, powerful mandate".
"You are an example of history which is changing... Your mandate is undoubtedly cancelling the bailouts of austerity and destruction.
"The troika for Greece is the thing of the past," he added, referring to the country's biggest international lenders - the European Union, International Monetary Fund (IMF) and European Central Bank (ECB).
He also promised to negotiate a fair and mutually beneficial financial solution.
Mr Tsipras earlier vowed to reverse many of the austerity measures adopted by Greece since a series of bailouts began in 2010.