Greece faces days, possibly weeks, of political instability after voters angry at crippling income cuts punished mainstream politicians.
They voted a far-right extremist group into Parliament but gave no party enough votes to govern alone.
With more than 99% of the vote counted, conservative New Democracy led with 18.9% and 108 of Parliament's 300 seats.
Party leader Antonis Samaras, who backs Greece's bailout commitments for austerity, will launch coalition-forming talks later today.
"I understand the rage of the people, but our party will not leave Greece ungoverned," said Mr Samaras.
But that could prove impossible because even with the support of the only other clearly pro-bailout party elected, Socialist Pasok, New Democracy would fall two seats short of a governing majority.
As first party, Mr Samaras will get three days to seek partners.
If he fails the mandate will go to the second party for a further three days, and then to the third party. If no agreement can be reached, the country heads to new elections.
Both Mr Samaras and Pasok leader Evangelos Venizelos, who spent nine months as finance minister, indicated any unity government would have to include more than just their two parties.
But in a note that will likely raise alarm among Greece's international creditors, Mr Samaras insisted any coalition should renegotiate the terms of the country's bailout.
Yesterday's big winner was the anti-bailout Radical Left Coalition, whose unprecedented second place gives it 52 seats.
Pasok, which has spent 21 years in government since 1981 and stormed to victory with more than 43% in 2009, saw its support slashed to about 13.5%. It will have just 41 seats, compared to 160 in the last election.
The two parties saw their support plummet to the lowest level since 1974, when Greece emerged from a seven-year dictatorship.
The outcome showed widespread public anger at the harsh austerity measures imposed over the past two years in return for rescue loans from other European Union countries and the International Monetary Fund. Without the funds, Greece faced a disastrous default that could have dragged down other financially troubled European countries and seen it leave the euro.
Voters who deserted the two mainstays of Greek politics in droves headed to a cluster of smaller parties on both the left and right, including the extremist Golden Dawn, which rejects the neo-Nazi label and insists it is nationalist patriotic.
The movement has been blamed for violent attacks on immigrants and ran on an anti-immigrant platform, vowing to "clean up" Greece and calling for land mines to be planted along the country's borders.
The party looked set to win about 7% of the vote, giving it 21 deputies in parliament - a stunning rise for a group that earned just 0.29% of the vote in 2009.
The other big winner was Alexis Tsipras, the 38-year-old leader of the Radical Left Coalition, or Syriza, who saw his party poised for an unprecedented second place with 16.4% and 51 seats - the first time in nearly 40 years that any party other than New Democracy or Pasok has held the spot.
Turnout stood at just over 64% - a low figure for the country, where voting is officially compulsory, although no sanctions are applied for not casting a ballot.