Ballots opened nationwide in Greece on Sunday for a critical general election that will determine whether the debt-laden nation will complete painful austerity reform or see its eurozone future jeopardised.

Some 9.8 million Greeks began voting at 04:00 GMT in what is expected to be a close contest between the pro-bailout New Democracy conservative party and the anti-austerity radical leftist Syriza party that has spooked European leaders.

Pressure has built on the struggling eurozone member over the past week to choose its next government carefully, with the prospect of a loan freeze that would imperil its weakened economy now seen as a real possibility.

European leaders have voiced concern and media in Germany, the eurozone member shouldering most of Greece's multi-billion EU-IMF bailout, have also weighed in with warnings ahead of the election.

In a twist of fate, Greece will likely take on Germany in Euro 2012 after advancing to the quarter-finals with a surprise victory over Russia on Saturday.

The radical left Syriza's firebrand leader Alexis Tsipras has vowed to tear up the rescue deal that has given Greece a credit lifeline in exchange for steep budget cuts and says he will renegotiate with creditors from scratch.

"It is extremely important that Sunday's Greek elections lead to a result in which those who form the government say, 'Yes, we want to keep to our commitments,'" German Chancellor Angela Merkel told a regional meeting of her party on Saturday.

A front-page call by the German edition of the Financial Times urging voters to 'Resist the demagogue (Tsipras)' sparked an angry reaction from Greek parties.

Germany's Bild newspaper added to tensions on Saturday with a tart letter telling Greeks their ATMs only had euros because "we put them there."

"If the parties who want to be through with austerity and reforms win the election and contravene every agreement, we will stop paying," it said.

"You have elections tomorrow but you don't have a choice," Bild added.

"You will choose between painful wisdom and total disaster."

Eurogroup chief Jean-Claude Juncker has also warned a radical left victory in Greece would have 'unpredictable' consequences for the eurozone as Greeks fed up with austerity prepared for Sunday's elections.

"If the radical left wins... the consequences for monetary union are unpredictable," said Juncker, who leads eurozone finance ministers and is also Luxembourg's Prime Minister, in an interview with the Austrian daily Kurier.

New Democracy leader Antonis Samaras wants a more moderate renegotiation of the deal and accuses Tsipras of playing with fire. He has warned that a vote for Tsipras could bring back the drachma currency.

No one party is expected to win enough votes to secure a majority in Parliament, and the days to come are likely to be dominated by coalition talks.

Analysts say that New Democracy would find it easier to form a coalition if it wins - although it might struggle to secure a strong majority in Parliament.

Syriza would find it harder to form a leftist coalition, analysts say.

Ballots will remain open until 16:00 GMT, with exit polls due immediately afterward and the first indicative results expected after 18:30 GMT.

It was not immediately clear when ballots would open in areas of rural Athens hit by wildfires on Saturday that burned down many summer homes and left three injured firefighters