They predicted the conservative New Democracy party would win 18.9% of the votes, meaning it was set to hold 108 seats in the 300-member parliament.
This was far less than the 151 needed to form a government.
The left-wing Radical Left Coalition, or Syriza, was expected be second with 16.8% and 51 seats.
The socialist Pasok, previously the largest party, was projected to slip to third place. It was set to win 13.4% of the votes, giving it 41 seats.
New Democracy and Pasok had been in a coalition implementing unpopular economic reforms as the country tackles its debt crisis. They now appear to have suffered big losses.
With no outright majority, another coalition government will have to be formed.
Antonis Samaras, as leader of the biggest party, New Democracy, will be given the task.
But observers say he will have difficulty forming an administration able to maintain Greece's austerity drive and implement more cuts demanded by the country's creditors.
Mr Samaras called for a coalition government with two aims: for Greece to remain in the euro and to change the terms of its international bailout.
Shadow foreign minister Panos Panagiotopoulos of New Democracy said a "strong earthquake" of public anger had struck his party and Pasok, allowing smaller anti-austerity groups to make major gains.
Syriza chief Alexis Tsipras called for the overturning of the bailout.
The results of the election could determine the course the country takes after years of austerity measures.
The spending cuts and tax rises angered voters but were critical in convincing international creditors to extend Greece's loans to keep its economy afloat.
Meanwhile, the extreme right-wing Golden Dawn party is set to win enough votes to enter parliament for the first time in nearly 40 years.
An exit poll suggested it will receive 6%-8% of the votes amid rising immigration and crime concerns.
This was well above the necessary 3% threshold.
Golden Dawn's policies include placing landmines on the Turkish border to stem immigration.
New Democracy will have three days to carry out coalition negotiations.
If the talks fail, 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 another election - a prospect which has alarmed Greece's international creditors.