The two top contenders in an election crucial for Greece, Europe and the world were in a dead heat, exit polls showed, with Greek voters apparently polarised over the harsh austerity measures demanded by international bailouts.

As central banks stood ready to intervene in case of financial turmoil, Greece held its second national election in just six weeks to try to select a new government after an inconclusive ballot on May 6. The outcome of Sunday's vote could determine whether Greece is forced to leave the joint euro currency, a move that could drag down other European countries and have potentially catastrophic consequences for the global economy.

An exit poll carried out by five polling companies for Greece's major television channels projected the conservative New Democracy party as coming in with between 27.5 and 30.5 percent of the vote, with the anti-bailout radical left Syriza party at between 27 and 30 percent.

Another exit poll carried out for a separate TV station projected Syriza as possibly being marginally ahead, at between 25 and 31 percent compared to New Democracy's 25 to 30 percent.

Neither party will have enough seats in the 300-member Parliament to form a government, meaning talks on forming a coalition government will almost certainly begin Monday.

The two parties vying to win have starkly different views about what to do about the €240 billion in bailout loans that Greece has been given by international lenders.

Syriza head Alexis Tsipras, a 37-year-old former student activist, has vowed to cancel the terms of Greece's international bailout deal and repeal its austerity measures - a move many think will force Greece to leave the 17-nation eurozone.

New Democracy leader Antonis Samaras says his top priority is to stay in the euro but has promised to renegotiate some terms of the bailout.

Whichever party comes first in Sunday's vote gets a bonus of 50 seats in Parliament.

Greece has been dependent on rescue loans since May 2010, after sky-high borrowing rates left it locked out of the international markets following years of profligate spending and falsifying financial data. The spending cuts made in return have left the country mired in a fifth year of recession, with unemployment spiraling to above 22 percent and tens of thousands of businesses shutting down.

There are no rules, however, governing a country's exit from the eurozone, and a Greek exit could spark a panic that other debt-strapped European nations - Portugal, Ireland, Spain and Italy- might also have to leave.

That domino scenario - known in economic terms as contagion - could engulf the euro, causing a global financial panic not unlike the one that gripped the world in 2008 after the investment firm Lehman Brothers failed in the U.S.

The vote also came after a difficult week for Spain and Italy, which saw their borrowing costs soar. Tens of thousands of Italian workers rallied Saturday in Rome to protest pension cuts, tax hikes and labour reforms.

Tsipras, has vowed to rip up Greece's bailout agreements and repeal the austerity measures, which have included deep spending cuts on everything from health care to education and infrastructure, as well as tax hikes and reductions of salaries and pensions.

But his pledges, which include cancelling planned privatisations, nationalising banks and rolling back cuts to minimum wages and pensions, have horrified European leaders, as well as many Greeks. Tsipras' opponents argue that the inexperienced young politician is out of touch with reality, and that his policies will force the country out of the euro and lead to poverty for years to come.

Virtually unknown outside of Greece four months ago, Tsipras and his party shot to prominence in the May 6 vote, where he came a surprise second and quadrupled his support since the 2009 election.

Journalists and television news crews from across the world jostled for space to see Tsipras vote Sunday in Athens.

"We have beaten fear. Today we open a road to hope," Tspiras said, adding that he was confident of victory. "Today we open a road to a better tomorrow, with our people united, dignified and proud. In a Greece of social justice and prosperity, an equal member of a Europe that is changing."

The young left-wing leader has accused his rivals of attempting to terrorise the population by casting him as the man who will ruin the country. He insists he will keep Greece within the euro - something that opinion polls have shown about 80 percent of Greeks want - but European leaders have made it clear there is no room for Greece to reject the bailout and stay in the eurozone.

Samaras, meanwhile, has cast Sunday's choice as one between the euro and returning to the drachma. He has vowed to renegotiate some of the bailout's harsher terms but insists the top priority is for the country to remain in Europe's joint currency.

"Today the Greek people speak. Tomorrow a new era for Greece begins," Samaras said after voting in southern Greece.

Separately, Greek police were investigating the discovery Sunday of two unexploded hand grenades outside private Skai television station on the outskirts of Athens. Greek government spokesman Demetris Tsiodras denounced the action as an attempt to spoil the smooth running of the elections.

Police also said they have notified Twitter about a forged message purportedly sent out by Greece's Communist Party urging voters to cast their ballots for Syriza.

Strong winds in the Greek archipelago also forced the cancellation of some ferry routes Sunday, raising doubts about whether some voters would be able to get to island polling stations.

Polling station in Athens attacked during election

Police also say about ten men armed with sledgehammers and wooden bats attacked a polling station wounding two policemen guarding it and setting fire to the ballot box.

No voters were hurt during the attack, which occurred half an hour before polls closed in the crucial election.

The attack - the only one reported so far at a voting station during the balloting - took place in the Athens neighbourhood of Exarhia, a traditional haven for leftists and anarchists near the city center.

Riding motorcycles and wearing helmets and balaclavas, the men fled the scene after smashing the ballot box and setting fire to it. Police say the two wounded officers were hospitalised.

- AP