Panos Kammenos and Alexis Tsipras share an opposition to Greece?s international bailout terms
The coalition will have a comfortable majority in Greece?s new parliament.
Syriza leader Alexis Tsipras has vowed to renegotiate Greece?s bailout, worth ?240bn (?179bn; $268bn), to end Greece?s ?humiliation and pain?.
Several European politicians have warned him to respect bailout terms.
The BBC?s full coverage of the reaction can be found here.
The euro recovered from an 11-year low against the US dollar as investors digested what Syriza?s victory means for the eurozone?s future.
Europe?s main share markets also rose ? after initial falls ? on hopes that a compromise over Greece?s bailout terms might be found.
With nearly all of the votes counted in Sunday?s poll, Syriza looks set to have 149 seats, just two short of an absolute majority. The Greek Independents are projected to have 13 seats in the 300-seat parliament.
The election result is expected to be one of the main issues at Monday?s meeting of 19 eurozone finance ministers.
Sunday?s result means that a majority of voters in Greece have essentially rejected a core policy for dealing with the eurozone crisis as devised by Brussels and Germany, the BBC?s Gavin Hewitt in Athens says.
The troika of lenders that bailed out Greece ? the European Union, European Central Bank, and International Monetary Fund ? imposed big budgetary cuts and restructuring in return for the bailout money.
But the man tipped to become the new Greek finance minister Yanis Varoufakis, told the BBC the austerity regime had been ?fiscal waterboarding policies that have turned Greece into a debt colony?.
The economy has shrunk drastically since the 2008 global financial crisis, and increasing unemployment has thrown many Greeks into poverty.
On Sunday, Mr Tsipras told jubilant supporters he wanted to write off half of Greece?s debt, but was ready to negotiate ?a viable solution? and wants the country to stay in the eurozone.
However, a number of European politicians have warned Greece?s new leaders to abide by the deals the country has signed.
Arriving for the meeting in Brussels, the chairman of the Eurogroup, Jeroen Dijsselbloem, told reporters: ?There is very little support for a write off in Europe.
?The most important thing is that if you remain in the eurozone you stick to the rules we have.?
Mr Dijsselbloem said the possibility of a further extension of the bailout ? which runs out at the end of February ? was on the agenda of the Eurogroup meeting.
German Chancellor Angela Merkel?s spokesman also said Greece had to stick to its previous commitments.
Belgian Finance Minister 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 flexibility.
UK Chancellor of the Exchequer (Finance Minister) George Osborne told the BBC Syriza?s promises would be ?very difficult to deliver, and incompatible with what the eurozone currently demands?.
Italy?s EU Affairs Minister Sandro Gozi sounded a more positive note, speaking of ?new opportunities to pursue change in Europe to create growth and investment and fight against unemployment?.
Source: BBC
