A new chapter in Greece’s uphill struggle to remain solvent – and in the eurozone – has begun in earnest as anti-austerity politicians assumed the helm of government following the radical left Syriza party’s spectacular electoral victory on Sunday night.

Ushering in the new era, Alexis Tsipras was not sworn in, as tradition dictates, in the presence of Archbishop Iernonymos but instead took the oath of office in a civil ceremony. At 40, he becomes the country’s youngest premier in modern times.

The leftist, who surprised Greeks by speedily agreeing to share power with the populist rightwing Independent Greeks party, Anel, was on Monday afternoon handed a mandate by president Karolos Papoulias to form a government following his investiture at the presidential palace. Afterwards, the new prime minister pronounced that he will give his all “to protect the interests of the Greek people”.

Earlier, Panos Kammenos, Anel’s rumbustious leader, emerged from hour-long talks with Tsipras saying the two politicians had successfully formed a coalition.

“I want to say, simply, that from this moment, there is a government,” Kammenos told reporters gathered outside Syriza’s headquarters.

“The Independent Greeks party will give a vote of confidence to the prime minister, Alexis Tsipras. The prime minister will go to the president and … the cabinet makeup will be announced by the prime minister. The aim for all Greeks is to embark on a new day, with full sovereignty.”

Facebook Twitter Pinterest How Greeks made history in one day - video

With 36.3 % of the vote, Syriza fell two seats short of the 151 MPs it needed to govern alone. The Independent Greeks, who have huge ideological differences with the leftists but are bonded by the desire to end biting EU-IMF-mandated cutbacks, won 4.75% of the vote and 13 seats. The conservative New Democracy party – the dominant force in a coalition lead by the outgoing prime minister Antonis Samaras – suffered ignominious defeat, collapsing to 76 seats in the 300-seat parliament.

Syriza’s victory could encourage other radical anti-austerity parties in southern Europe, including Spain’s Podemos, whose leader, Pablo Iglesias, told a rally in Valencia: “Hope is coming, fear is fleeing. Syriza, Podemos, we will win.”

After five years of austerity-fuelled recession that has driven the vast majority of Greeks into poverty and despair, Syriza cadres described the new administration as a “national salvation government”.

The veteran leftist Dimitris Vitsas, who sits on Syriza’s political secretariat, said now that the “people have spoken” the government would move quickly to tackle the devastating effects of austerity. “Our immediate and most pressing priority is to alleviate the humanitarian crisis,” he said in an interview on SKAI TV. “The Greek people have spoken, they have cancelled the policies of the memorandum,” he said referring to the deeply unpopular bailout accords debt-crippled Athens has signed with creditors.

With more than 26% of the population out of work, and more than a third at risk of poverty, the new government was assuming the reins not only of a broke state but a broken society.

“And yet despite all this talks of economic catastrophe, [with the advent to power of a leftwing government] “we have woken up to a sunny day,” Vitsas said. “The banks haven’t closed, they are operating normally, schools are open. Everything is just as it should be.”

Facebook Twitter Pinterest Optimism and uncertainty on the streets of Athens following Greek elections – video

But Tsipras faces a formidable task, and within hours of his victory, foreign and European officials pressed home the point that there would be little wiggle room – and not much of a grace period – for the new government.

The leftists say full economic recovery can only come if Athens’ bailout agreements are rewritten and the country’s monumental debt pile – totalling €318bn (£238bn) – is reduced.

Yanis Varoufakis, the internationally renowned economist, newly installed as a Syriza MP and tipped to become the finance minister, likened the Greek economy to a “poisoned chalice”.

“Fiscal waterboarding has turned us into a debt colony,” he told the BBC.

Before Sunday’s election, Varoufakis, who gave up his post at Texas University in Austin to run with Syriza, had described the prospect of resuming stalled talks with foreign lenders as daunting.

“It is an extremely scary project and prospect,” he said. If, as looks likely, the academic assumes the finance ministry portfolio, he will almost certainly head the negotiations.

Yet Greece’s partners show no signs of letting up. “There are internal eurozone rules to be respected,” IMF chief Christine Lagarde told Le Monde. “We cannot make special categories for such or such country.”

Arriving in Brussels for a scheduled meeting of euro group finance ministers, Pierre Moscovici, the European commissioner for economics and finance, told reporters that the EU expected Greece to pay back its debt. “We all want a Greece that stays on its feet, creating jobs and growth, reducing inequality, and a Greece that repays its debt.”

Germany has pledged to work with Greece’s new government, but has not given any hint that it might support a debt deal. In a press conference on Monday, government spokesman Steffen Seibert said: “The Greek voters have chosen Syriza and we have to respect that … Currently, we have a result and no government. But when we do we will offer to work together with them.”

However, Seibert made clear that any talks would not be taking place between Germany and Greece, but between Greece and its European partners.

Analysts in Athens warned that with the new government poised to ratchet up tensions with creditors, Greece was headed for a prolonged – and possibly fatal – period of political uncertainty. The country, which faces debt repayments of €4.3bn in the coming months, has been guaranteed bailout funds only until the end of February.

The unexpectedly good showing of the neo-Nazi Golden Dawn party – with 6.28% of the vote the extremists emerged as the country’s third biggest political force – added to the fears. “Greece is for sure entering a new era and my great worry is that it could be very destructive,” said the prominent commentator Alexis Papahelas. “There are people in this country who know what war and occupation means but the young don’t seem to know,” he said. “My fear is that they may turn to an even more anti-systemic party, on the far right [in the event of failure]. I really don’t rule it out.”