German Chancellor Angela Merkel said it was "extremely important" for Greeks to elect lawmakers who would respect the terms of a controversial bailout deal, which Syriza leader Alexis Tsipras has promised will be "history" on Monday.
Eurogroup chief Jean-Claude Juncker also warned on the eve of the vote that choosing Syriza could have "unpredictable" consequences for the eurozone as international markets watched the momentous vote with bated breath.
Polls open at 0400 GMT and close at 1600 GMT, with exit polls also due out then and the first indicative results expected at around 1900 GMT in the second election in six weeks after a vote on May 6 failed to produce a government.
Germany's Bild newspaper added to tensions ahead of the vote with an open 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.
Tsipras, a 37-year-old former student activist from a working-class Athens district, is running neck-and-neck in the polls with Harvard-educated Antonis Samaras, the 61-year-old head of the conservative New Democracy party.
"It's true we want to blow up a system that isn't working but I'm afraid Tsipras might turn out just as incapable as the others and that the situation might continue to get worse," said Dora Fotopoulou, a 48-year-old psychologist.
Polls show an overwhelming majority of Greeks want to keep the euro. In their 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.