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.
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 Euro zone 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 Euro zone 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.
Addressing his jubilant supporters in front of Athens' university, 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.
For his part, Mr Samaras said earlier: The Greek people have spoken and I respect their decision, pointing out that he had inherited a hot potato on coming into office and that he and his party had done much to restore his country's finances.
The result is being closely watched outside Greece, where it is believed a Syriza victory could encourage radical leftist parties across Europe.
There is an ongoing thriller surrounding the absolute majority, said Michalis Karyotoglou, head of Singular Logic, the software group monitoring the voting process for the interior ministry.
Either way however, partial results from Greece's election commission showed a clear Syriza lead.
With most votes counted, Syriza is polling 36%, while New Democracy is on 28%.
Another five parties - including the far-right Golden Dawn and centrist The River - are expected to be represented in the 300-member parliament.
The proportion of votes won by smaller parties will have a large impact on whether Syriza can gain the required 151 parliamentary seats to govern with an absolute majority.