The conservative New Democracy party appeared the most likely to win the top spot, while the extreme right-wing Golden Dawn seemed set enter parliament for the first time, gaining several seats - a meteoric rise for a party on the fringes of politics until a few months ago.
Days of talks are likely to ensue as parties attempt to hammer out a governing coalition.
The election will determine the country's course after years of austerity measures that have outraged voters but which were critical in convincing international creditors to extend Greece billions in loans to keep its debt-saddled economy afloat.
Greece is heavily dependent on billions of euros worth of international rescue loans from other European countries and the International Monetary Fund, and must impose yet more austerity measures next month to keep the bailout funds flowing and prevent a default and a potentially disastrous exit from the group of nations that use the euro currency.
According to updated exit poll figures an hour and a half after polls closed, New Democracy was projected to win 19-20.5 percent, followed by the leftist Radical Left Coalition, or Syriza, with 15.5-17 percent. The formerly majority socialist PASOK party was projected in third place with 13-14 percent.
"The truth is here, the reality of this result is that at the moment this produces no government," outgoing deputy prime minister and senior PASOK official Theodoros Pangalos said. "It is not a question at the moment of who gets a little more or a little less." Initial official results with 14.4 percent of the vote counted showed New Democracy with 21.2 percent, Syriza with 14.99 percent and PASOK plunging to 14.83 percent.
The outcome is a devastating blow to PASOK, which won a landslide victory in the last parliamentary elections in 2009 with more than 43 percent. PASOK, along with New Democracy, have dominated Greek politics since the fall of the seven-year dictatorship in 1974.
"This is a major political earthquake, that has devastated PASOK," senior New Democracy official Panos Panagiotopoulos said. "New Democracy remains the first party but has a very low support number. It is an explosion of anger and despair. The fallout has affected many parties fairly and unfairly."
Thirty-two parties were vying for the support of nearly 10 million registered voters, many of whom were undecided on the eve of the election.
Golden Dawn, which has vowed to kick out immigrants and mine Greece's borders with Turkey, was predicted to win between 6.5-7.5 percent, well above the 3 percent needed to enter parliament. If borne out by official results, it will be a stunning result for a party that won just 0.23 percent in the 2009 elections.
"Greek citizens should not fear us, the only ones who should fear us are the traitors," Golden Dawn leader Nikolaos Michaloliakos told The Associated Press.
Whichever party wins the most votes will get a bonus of 50 seats in the 300-member parliament. But with percentages so low and between seven and 10 parties projected to enter parliament, that will not be enough to form a governing majority of 151 seats.
The first party will be given a mandate to form a coalition, and will have three days for negotiations. If it fails, the mandate will go to the second party for a further three days, and then on to the third party.
If no coalition emerges, the country will have to have another election, a prospect which has alarmed Greece's international creditors.
New Democracy and PASOK were uneasy coalition partners in a temporary six-month power-sharing deal forged last November following a political crisis that saw former PASOK head George Papandreou resign as prime minister. In 2009, Papandreou had led his party to a landslide election win, earning 43.92 percent of the vote.
Before the election, New Democracy head Antonis Samaras had vowed not to form a coalition with its socialist rivals, saying such a prospect would require too much haggling to be effective. However, it was not clear how he would react to the official result.