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 investment firm UBS has predicted the Greek economy would collapse, contracting up to 50 percent the year after it returned to its old currency, the drachma. The move could also cause Standard & Poor's 500 index to drop 15 percent in weeks, says Paul Christopher, the chief international strategist for Wells Fargo Advisors.
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 canceling planned privatizations, nationalizing 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.
"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 terrorize the population by casting him as the man who will ruin the country. He insists he will keep Greece within the euro 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.
Earlier Sunday, about 10 men armed with sledgehammers and wooden bats attacked a polling station in central Athens, wounding two policemen guarding it and setting fire to the ballot box shortly before polls closed. The attack — the only one reported so far — took place in the Athens neighborhood of Exarhia, a traditional haven for leftists and anarchists.
Greek police were also 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 devices as an attempt to spoil the smooth running of the election.
Menelaos Hadjicostis, Demetris Nellas and AP television in Athens contributed.
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