The EU and Germany have stressed Greece must keep to the terms of the two EU/IMF bailouts, after a surge of voter support for anti-austerity parties.
The two main parties, New Democracy and Pasok, attracted less than a third of the vote, in an election plunging Greece into political uncertainty.
Chancellor Angela Merkel said Greece's reforms were of "utmost importance".
New Democracy leader Antonis Samaras will now face a struggle finding parties prepared to join a government.
With about 99% of votes counted, centre-right New Democracy (ND) is leading with 18.9%, down from 33.5% in 2009.
New Democracy will try to form an austerity-supporting pro-European coalition government, perhaps with a third party, because it would not gain enough with Pasok to form an absolute majority.
But the anti-bailout party Syriza will also try to form an alternative coalition government. There could be a clash of the two - we could be facing fresh elections within weeks.
This country is now placed into a period of intense political instability - and by extension the eurozone as a whole.
A majority of Greeks have voted against the bailout and against the austerity, which will make it very difficult for the EU or IMF to call for yet more austerity here.
The success of the new-right Golden Dawn party indicates how comprehensive a rejection of the political mainstream, the bailout and austerity there has been.
The stability and the future of Greece are now in doubt once again. That will bring a lot of dismay to the financial markets and to the eurozone as a whole.
Euro declines on election results
A radical left coalition, Syriza, came second with 16.8% and a party of ultra-nationalists - Golden Dawn - polled almost 7%.
Speaking to reporters on Monday, German government spokesman Steffen Seibert spelt out Berlin's position that "the agreed programmes must be adhered to". He added that Germany would support Athens in returning to competitiveness and financial stability, "whatever its government is".