UPDATE: Second exit poll shows New Democracy further ahead.
Via Antenna TV: New Democracy - 28.6-30%, Syriza - 27-28.4%, Pasok - 11-12.5%, Independent Greeks - 6.8-7.8%, Golden Dawn - 6.5-7.1%, Democratic Left - 5.8-6.6%, Communists - 4.8-5.6%
The Conservative New Democracy party and left-wing Syriza party are neck-and-neck in the Greek election, according to early exit polls.
The poll, broadcast on state-run NET TV, said New Democracy have 27.5%-30.5%, Syriza have 27%-30%, while the Socialist Pasok party were on10%-12%.
Syriza has run on a platform of opposing the European bailout, while New Democracy is more supportive of the austerity demands of the eurozone.
It was hoped that the election, the second in six weeks, would lead to workable majority for one of the parties, however the exit polls suggest that the country could be in for another period of instability.
An uncertain result will do little to quell unease among European leaders fearful that an anti-austerity administration will lead the failing nation to crash out of the eurozone.
It is also unlikely to appease jittery stock markets desperate for certainty when they open for trading on Monday.
Lord Mandelson warned Greece "faces a very long and a very painful road back" and called on European leaders to take swift action to tackle wider economic turmoil in the single currency.
Former prime minister Tony Blair said the only way the euro was likely to survive, regardless of Greece's future within it, was for Germany to throw its financial weight behind the currency.
Earlier this week, Chancellor George Osborne warned Britain faces dire consequences if Greece leaves the eurozone without an "ambitious" plan to deal with the fallout.
Parties potentially face a fresh round of coalition talks in the hope of forming an administration. Failure could mean a third round of elections later this year.
German chancellor Angela Merkel has warned Greece it cannot dodge austerity measures by renegotiating its bailout agreement.
She said: "We will have to speak to any government. I can only warn everyone against leaving the currency union. The internal cohesion of the eurozone would be in danger."
Central banks, including the Bank of England, Bank of Japan and US Federal Reserve, are expected to launch emergency support measures to cushion the blow of an implosion in the eurozone.
A chaotic Greek exit from the single currency would fuel a run on the banks, with the population likely to want to avoid having its euros converted to drachma or savings frozen.
The same could happen to banks in countries which have lent heavily to Greece, such as France, triggering a domino effect known as contagion, which could spread to banks in non-eurozone countries, including the UK.
Prime Minister David Cameron will fly out to Mexico overnight for the G20 summit of leading nations where the ramifications of Sunday's result are expected to be top of the agenda. He will be in touch with London via satellite phone.
The German chancellor, French president Francois Hollande and Italian premier Mario Monti are expected to delay their travel to Los Cabos until tomorrow to be in place to deal with the immediate aftermath of tonight's results.
Greek Elections: Angela Merkel Warns Country Cannot Renegotiate Bailout
Greeks go to the polls: