Left-wing anti-austerity party Syriza has won Greece's general election, putting the country on a possible collision course with the European Union (EU) over its massive bailout.
With nearly 75 percent of the votes counted, Syriza is projected to win 149 seats, just two short of an absolute majority, though that number could change, the BBC reported.
Party leader Alexis Tsipras, who has vowed to renegotiate Greece's debt with international creditors, said "today the Greeks wrote history".
The ruling New Democracy has come a distant second.
Outgoing Prime Minister Antonis Samaras has admitted defeat and phoned Mr Tsipras to congratulate him.
Syriza's result is expected to send shockwaves through Europe, BBC correspondent Gavin Hewitt said.
A majority of voters in Greece have essentially rejected a core policy for dealing with the eurozone crisis as devised by Brussels and Berlin, Mr Hewitt added.
'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 EU, 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.
"I hand over a country that is part of the EU and the euro. For the good of this country, I hope the next government will maintain what has been achieved."
The result is being closely watched outside Greece, where it is believed a Syriza victory could encourage radical leftist parties across Europe.
Deutsche Bundesbank president Jens Weidmann said he hoped the new Greek government would not jeopardise what had been achieved to solve the country's economic crisis.
The head of Germany's central bank said agreements had to be honoured.
Germany's Left Party called the result a sign of hope for a new start in Europe and the leader of the Spanish anti-austerity party Podemos said the clock was ticking for change in Spain.
The partial results from Greece's election commission showed a clear Syriza lead.
With most votes counted, Syriza is polling 36 percent, while New Democracy is on 28 percent.
Another five parties - including far-right Golden Dawn and centrist The River - are expected to be represented in the 300-member parliament, beating the three percent threshold.
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.
- BBC / Reuters