Eurogroup chief Jean-Claude Juncker 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.
His comments were echoed in Germany by Wolfgang Bosbach, a leading lawmaker and close ally of Chancellor Angela Merkel, in excerpts from an interview with Frankfurter Allgemeine Zeitung to be published in full on Sunday.
"If the radical left carries on saying it wants the help of all the other countries in the eurozone but does not offer anything in return, then it will only be a matter of time before Greece exits" the euro, Bosbach said.
The radical left Syriza's firebrand leader Alexis Tsipras has vowed to tear up a bailout deal that has given Greece a credit lifeline in exchange for steep budget cuts and says he will renegotiate with creditors from scratch.
"The bailout deal is already in the past. It will be history for good on Monday," the 37-year-old said this week. He has given himself a 10-day deadline for negotiations -- in time for a summit of European leaders on June 28 and 29.
The Syriza party chief says the mood in Europe is shifting against austerity and that the European Union and International Monetary Fund will not want to risk a Greek euro exit that would send shockwaves through the global economy.
At his final election rally in Athens, Tsipras accused his rival, conservative New Democracy leader Antonis Samaras, 61, of defending "Merkel's Europe of the past".
"We guarantee the Europe of the future," he said.
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.
At his last campaign rally, Samaras said: "We will exit the crisis. We will not exit the euro. We will not let anyone take us out of Europe."
In public comments at least, European leaders warn that Greece must respect its international debt commitments or risk leaving the euro club, and the EU and the IMF have suspended loan payments until after the elections.
IMF chief Christine Lagarde said talks with a new government in Greece must resume next week.
"We do not really know what has been implemented, what has been respected or not in the last six or eight weeks," she told French daily Liberation, and underscored the need for Europe to strengthen the eurozone.
"In the very short term, perhaps in less than three months, it is necessary for Europeans, particularly those in the heart of the eurozone, to give strong signals about their collective will to buttress their monetary union," she said.
Though the German government has until now refused to endorse anyone in the Greek race, Merkel on Saturday said it was vital for Greece to elect lawmakers who will respect austerity commitments.
Merkel and French President Francois Hollande, recently at odds over how to deal with the crisis, held "constructive and fruitful" telephone talks Saturday, Hollande's office said.
Spanish Prime Minister Mariano Rajoy meanwhile said he was confident Greece would stay in the euro and "respect its engagements."
Greece has already been forced to seek bailouts twice, first for 110 billion euros in 2010 and then for 130 billion euros this year plus a 107-billion-euro private debt write-off -- for a total of 347 billion euros ($439 billion).
For many Greeks a fine-tuning of the terms of the loans may not be enough as public anger is rising against the steep pay and pension cuts seen since the crisis first exploded in 2009, setting off a chain reaction across Europe.
Greece is now in its fifth year of recession, and many young Greeks are voting with their feet by emigrating, while local media reports warn that the state will run out of cash to pay public-sector salaries and pensions on July 20.
"We want the euro, but we also have to live. We can't sacrifice everything," said Syriza supporter Meri Primi, a 45-year-old teacher in Athens whose salary has fallen by 400 euros over the past two years to 1,100 euros a month.
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.
Polls open at 0400 GMT on Sunday and close at 1600 GMT, with exit polls due immediately afterward and the first indicative results expected after 1830 GMT.