Athens - After five years of extreme austerity prescribed to treat an epidemic of debt, a battered but defiant Greece on Sunday rejected the medicine.

With millions of voters turning out - from graffiti-scarred lanes in the Parthenon's shadow to islands shores lapped by aqua-green waves - the country delivered a historic win to Syriza, a radical leftist party that could put Greece on a collision course with the rest of Europe. The expected showdown has already rattled Greek financial markets and may challenge the core principle behind Europe's currency union.

As party supporters cheered in the streets of Athens, an official projection released Sunday evening showed that Syriza had secured just over 36 percent of the vote, well ahead of its nearest rival but just short of the support needed to form a government without relying on a coalition partner. Late Sunday, Prime Minister Antonis Samaras called the party's charismatic young leader, Alexis Tsipras, to concede.

Tsipras had campaigned on an upbeat if improbable platform of reviving the country's beaten-down economy by turning on the taps of government stimulus. He vowed to hire back laid-off workers, raise the minimum wage, expand access to health insurance and provide electricity to those who can no longer afford it.

Those pledges resonated powerfully in a nation where a third of the population lives at or below the poverty line, following years of cuts and recession. But the election of the first radical leftist government in the history of the European Union represents a provocative challenge to the international creditors who bailed Greece out to the tune of $284 billion on the condition that the country rein in its bloated costs.

Once in government, the party has said, it will demand a renegotiation of the bailout terms and cancellation of a sizable chunk of Greek's debt. The creditors, dominated by Germany, are unlikely to yield easily - raising fears of a showdown that ends with Greece exiting the euro, imperiling the rest of the currency union along the way.

"The whole idea of the euro is its irrevocability, and the moment you break that, you have no euro any more," said George Pagoulatos, a professor at the Athens University of Economics and a former government adviser. "This is a path down which no one wants to go."

And yet, Greek voters appeared ready to take the risk. Many said on Sunday they felt their country has hit rock bottom five years after it was forced to turn to a troika of international creditors for a massive bailout. Greece has little to lose, they said, by gambling on a party promising radical change.

"The way things are right now, it's impossible to survive," said Helen Zorba, a 55-year-old civil servant who said her pay has been cut by 20 percent in recent years while her taxes have gone up. "People are losing their dignity. You shouldn't have to go to the soup kitchen in order to live."

As the public's desperation has risen, so too has Syriza's popularity, with middle-class voters abandoning the centrist parties that have dominated Greek politics for decades and casting their lot with a group dominated by Marxist university professors and Communist activists.

"People say Syriza doesn't have the experience to govern. But the people who have the experience and the know-how to govern are the ones who failed Greece," Zorba said.

The official projection released on Sunday evening showed Syriza had comfortably defeated its nearest rival. Because the top party gets an additional 50 seats in the country's 300-member Parliament, it was considered possible, though not certain, that Syriza could form a government on its own. The government's projection, which was based on a partial count of votes, showed Syriza taking 150 seats - one short of a majority.

The incumbent party, the center-right New Democracy, was a distant second in the projected results, with 28 percent of the vote. A smattering of smaller parties, including neo-fascists, centrists, Communists and socialists were jostling for third, with each taking around five percent.

If Syriza does not secure a majority, it will need to form a coalition with another party, with the centrist The River and the nationalist Independent Greeks considered most likely and negotiations expected to unfold at a rapid clip in the coming days.

As results rolled in, hundreds of cheering Syriza supporters took to the streets of central Athens, waving flags and singing along as revolutionary anthems blared from loudspeakers.

Sunday's vote was Greece's third since 2012, and it came after the current government lost what amounted to a no-confidence measure in December.

In the campaign's closing days, Samaras warned Greeks that leaving the path of austerity and challenging the country's European creditors would lead the country to ruin.

"Syriza isn't going to change Europe. It will turn Europe against Greece," the prime minister told his party faithful in the campaign's final rally, on Friday evening.

His backers took the message to heart, with many saying on Sunday that they fear a Syriza-led government will crash the economy just as it has begun to show faint signs of improvement.

"It will be a catastrophe," said Anna Ventouri, a 45-year-old supermarket cashier who cast her ballot for the incumbents. "Tsipras says you can renegotiate the bailout, but it's a big lie. It's logical that if someone gives you a loan, they're going to want that money back."

In Greece's case, that someone is German Chancellor Angela Merkel. A champion of fiscal rectitude who preaches the gospel of austerity for all that ails Europe, she is considered unlikely to grant Tsipras concessions that could embolden other emerging leftist movements across debt-ridden southern Europe.

In Syriza's final rally in Athens before the vote, Tsipras stood arm-in-arm with the leader of a Spanish leftist party, Podemos, that has been surging in the polls. From the crowd, leftist Italian activists cheered them on.

"Merkel's a fanatic," said Anastasia Vasdeki, a 59-year-old Syriza voter who said she has had her pension slashed in recent years because of German-mandated austerity. "But she's becoming more and more isolated in Europe."

Although both sides have softened their rhetoric in recent weeks, German leaders have also hinted at a tough line in any talks, with leaks to the press that they are prepared to see Greece leave the euro if a Syriza government tries to renege on its bailout commitments.

Syriza leaders, meanwhile, have been adamant that they do not intend to take Greece back to its pre-2001 currency, the drachma. But they have also said there is no backup plan if European negotiators refuse to budge in canceling Greek debt.

"It's a game of chicken," said Pagoulatos, the economist, in which "no one intends to crash."

And yet, a crash could still come. The smart move for Europe, Pagoulatos said, would be to co-opt Tsipras by giving him enough of what he wants to neutralize his assault on austerity.

But another option, he said, is for Merkel to dig in, and turn Greece's leftist experiment "into a cautionary tale."

It's also possible that Syriza will stand in the way of a deal, with its disparate factions unable to agree on any package of concessions that falls short of the lofty promises made on the campaign trail.

"The rank-and-file has a component that is very radicalized," Pagoulatos said. "They are not ready to accept the kind of compromise that the situation requires, especially after generating such expectations for a full rupture with the past."