Voting opened on Sunday in a Greek election that could decide whether the heavily indebted country remains in the euro zone or heads for the exit, potentially unleashing shocks that could break up the single currency.

In an election fought over the punishing austerity package demanded by international lenders as the price of keeping Greece from bankruptcy, opinion polls showed the radical leftist Syriza party, which wants to scrap the deal, running neck and neck with the conservative New Democracy, which broadly backs it.

The European Union and International Monetary Fund have insisted that the conditions of the 130 billion bailout accord agreed in March must be accepted fully by a new government or funds will be cut off, driving Greece into bankruptcy.

Sunday's election is a re-run of an earlier vote on May 6 which produced no clear winner and forced voters back to the polls for a vote that is seen as a referendum on the future of the single currency.

Opinion polls show Greeks, weary after five years of deep recession, overwhelmingly favour remaining in the euro. But there is bitter anger over the repeated rounds of tax hikes, slashed spending and sharp cuts in wages.

'I voted with a heavy heart for a pro-bailout party because I want the country to stay in the euro, with the help of our European partners. I don't think the failed recipes of the left would get us out of this mess,' said Stratos Economou, 49, who runs a bakery shop.

All parties say they will keep Greece in the single currency but Tsipras believes the agreement can be renegotiated without Greece having to leave, betting that European leaders cannot afford the turmoil that would be unleashed by cutting a member of the euro zone loose.

On the right, establishment heir and New Democracy leader Antonis Samaras says rejection of the EU/IMF bailout would mean a return to the drachma and even greater economic calamity.

Samaras told supporters on Friday they faced a stark choice - 'euro versus drachma.'

Polls opened at 7.00 am (0400 GMT) and close at 7 pm. Exit polls will follow soon after voting ends.

Neither party is expected to win outright, triggering coalition negotiations with smaller parties. A new government would buy time, but little respite. Whoever comes to power may find their tenure is short-lived.

Central banks from Tokyo to London are readying arsenals to defend banks and national currencies against any post-election turmoil. The result will dominate a meeting of the Group of 20 world economic powers on Monday and Tuesday in Mexico.

European leaders weighed in on the eve of the vote, some of them openly urging Greeks to reject SYRIZA or risk undermining the very foundations of the single currency.

Finance officials in the euro zone have discussed limiting the size of withdrawals from ATM machines, imposing border checks and introducing euro zone capital controls as a worst-case scenario.

A Greek exit from the single currency would heap further pressure on two far larger European economies - Spain has already received up to 100 billion euros to save debt-riddled banks and Italy could be next to seek a bailout.

Euro zone officials have hinted they might give a new Greek government some leeway on how it reaches debt targets set by the EU/IMF bailout package, but there would be no change to the targets themselves.

Euro zone paymaster Germany warned Greeks on Saturday the bailout would not be renegotiated.

'That's why it's so important that the Greek elections preferably lead to a result in which those that will form a future government say: 'Yes, we will stick to the agreements',' Chancellor Angela Merkel told a party conference of her Christian Democrats.

Anger with the establishment parties of New Democracy and the Socialist Pasok propelled Syriza and its youthful leader, a former Communist student protest leader, from the obscure radical fringe to shock second place on May 6.

'The memorandum of bankruptcy will belong to the past on Monday,' Tsipras, 37, told his final election rally on Thursday, though analysts suggest the Syriza leader might temper his stance if confronted with the reality of leaving the euro.

The neo-Nazi Golden Dawn party also won seats in the first election, underscoring the fragmentation of a stressed society wrestling with unemployment of almost 23 percent and plummeting living standards.

Five opinion polls published before a blackout two weeks ago put New Democracy narrowly ahead. Two other polls had Syriza leading.

'It's obvious the country is now staring into the abyss,' leading Greek daily Kathimerini said in a front-page editorial on Sunday, calling for the creation of a New Democracy-led 'unity' coalition to keep the country in the euro.

But analysts say Samaras, 61, will find it hard to govern for long with an empowered Syriza protesting at the gates. Tsipras, if he wins, will inherit a country on the verge of bankruptcy. He has ruled out a government of national unity and promised to nationalise banks and halt privatisations. - Reuters