Within minutes of the Athens parliament failing to elect a president, Alexis Tsipras, the man most likely to become Greece’s next prime minister, said the country had experienced “a historic day”.

For the first time since the foundation of the modern Greek state nearly 200 years ago, radical leftists – marginalised, tortured and tormented for the best part of the 20th century – were on course to assume power. “Greeks,” he said, “should rejoice.” The government that had put the country through an assault course of austerity would soon be over.

The onerous terms of the deeply unpopular “memoranda”, agreed with foreign lenders to keep insolvent Greece afloat, would be overturned.

“The future has already begun,” he told reporters as the nation braced for early elections, a constitutional diktat when presidential polls fail. “You should be optimistic and happy.”

But the prospect of renewed political turmoil in the country where the eurozone crisis began will bring anything but a smile to the faces of decision makers in Brussels and Berlin. Greece, ineluctably, is being drawn into a new dance of uncertainty, a rollercoaster ride of high-pressure politics.

After six years of recession, the nation is only now beginning to show the first signs of recovery, posting a primary surplus – before interest payments on its mountain of debt – and returning to the capital markets that cut off funding at the start of the crisis.

But the country cannot survive alone. And market reaction to Monday’s news was instantly pessimistic, with the Athens Stock exchange plummeting by more than 10 percent.With bailout funds guaranteed to the end of February but far from assured after that, the spectre of Greece defaulting on its loans, and possibly crashing out of the eurozone, have been revived.

Athens faces debt repayments of up to €20bn (£15.6bn) in 2015. On Monday the Greek finance minister, Gikas Hardouvelis, said local banks had enough funds to survive until March but unless further financial assistance was agreed they would enter into the uncharted waters of non-liquidity after that. This makes an effectively rudderless Athens in the run-up to elections on 25 January more daunting for the “troika” of creditors at the EU, ECB and IMF.

“What we are looking at is a Greece in crisis,” said Dimitris Keridis, professor of political science at Panteion University. “The first party will be Syriza but the elections may well be inconclusive. And then, once a government is formed, it will have a few weeks to elect a new president and negotiate with the troika.”

The radical leftists wasted no time, announcing that Tsipras would hold his first pre-election speech in central Athens on Monday night. The result of the presidential poll indicated the thirst, at least on a political level, for new beginnings. Opinion polls at the weekend confirmed Syriza’s popularity among a population exhausted by austerity.

An Alco poll published in Proto Thema on Sunday showed the neo-Marxists leading by 3.3%.

Many supporters are neither leftist, nor admirers of Syriza’s anti-capitalist rhetoric, but Greeks appalled by the catastrophic effects of policies that have left 1.5 million unemployed, 3 million facing poverty and the vast majority unable to pay their bills.

The party has pledged to fight such ills – referred to as Greece’s silent humanitarian crisis – by renegotiating the bailout accords that have propped up the economy to the tune of €240bn.

“Solution to Greece’s problems lies within the European context but the first thing we will say is that the programme has failed because it was badly planned and didn’t see the pitfalls that were coming,” said Euclid Tsakalotos, Syriza’s shadow finance minister. “Then we will address the priorities of austerity and debt.”

The economist, the scion of a prominent Greek family who was raised in Britain and educated at Oxford University, denies the charge of populism that has been levelled at the leftists.

Outlining his economic policy, Tsipras, who turned 40 this year, promised instant debt relief, food relief and provision of homes and electricity for Greeks hardest hit by belt-tightening.

“The accusation is not right,” said Tsakalotos. “We are not promising everything to everyone. The [relief] measures will not affect anyone earning above €1,000 a month.”

Patronage politics and vested interests had made it impossible for Greek governments, including prime minister Antonis Samaras’s fragile, two-party alliance, to step back and reform.

“We are a new lot who can clean the slate and start again,” he told the Guardian. “All too often the troika has demanded irrelevant reforms at the expense of dealing with the important issues of corruption and tax evasion.”

Political analysts downplay the dangers posed by Syriza. As it has edged ever closer to power, the party has launched a concerted campaign to reassure and placate creditors of its policies and intent.

Tsipras has repeatedly said that the leftists are committed to Greece remaining in the euro currency bloc. But few deny that if it does come to power, Athens appears headed towards a major collision with its partners.

“There is potential for an accident because the new government will have to resume negotiations with the troika and complete them by the end of February,” said Pavlos Tzimas, a leading political commentator. “All the signs are that some sort of Syriza government will be handed the hot potato.”

In a hint of the looming clash, the German finance minister Wolfgang Schäuble said that, in the event of elections, whoever assumed power would have to respect agreements signed by those in office before. “The tough reforms are bearing fruit and there is no alternative to them,” he said.