On his first day as Greek prime minister after a historic election win that sent both hope and jitters rippling across Europe, Alexis Tsipras did everything differently.

Wearing an open collar at the grand presidential residence, the youngest Greek leader in 150 years skipped the traditional religious oath on a Bible. Instead of the customary handoff with his predecessor, the former Communist activist laid flowers at a memorial to hundreds of leftist resistance fighters slaughtered during World War II.

And then, insisting there was no more time for pageantry, the 40-year-old got to work. “We must expedite procedures,” the leader of the Syriza party told the elderly president during the blink-and-you-miss-it swearing-in, “as we face an uphill struggle.”

The down-to-business nature of the day reflected both the personality of the young radical who has vowed to transform Europe and the urgency of the moment for his desperately flailing nation.

Tsipras has promised to govern in a completely new way — but it remains to be seen whether that way will lead Greece to salvation or to ruin. The answer will matter not only for Greece but also for Spain, Italy and Portugal — deeply indebted nations across southern Europe that are flirting with their own insurrections against the long reign of austerity.

Critical decisions will need to come in Tsipras’s first weeks on the job, with Greece fast running out of money and in need of an infusion from its creditors to avoid a run on its banks before the end of next month.

In fiery campaign speeches, Tsipras crusaded against those creditors, winning over despondent Greeks with his vision of an economy unshackled from the relentless cuts that came as a condition of $284 billion in bailouts. Unless the deals were fundamentally renegotiated, he said, Greece would stop paying its debts, which still amount to a staggering 175 percent of gross domestic product.

But with Greece’s German-dominated paymasters showing no sign of backing down on Monday, Tsipras could soon be forced to make a fundamental choice between his lofty ideals and the reality of running a government that badly needs cash.

“For a Syriza government that has promised to end the bailout, if the first thing it does is extend the bailout, that’s clearly a problem for them,” said Nick Malkoutzis, a commentator with the political-analysis firm Macropolis. “But they don’t have a lot of options at the moment.”

Syriza will enter negotiations with Europe alongside an unlikely coalition partner, a fringe right-wing party called the Independent Greeks. Although Syriza won a convincing victory Sunday, the party fell just short of the threshold for a majority in the 300-seat Parliament. The two parties have virtually nothing in common beyond their shared antipathy for the stringent terms of the country’s bailouts.

The speed with which Greece’s economy could unravel if the new government can’t strike a deal for more cash was underlined within hours of Tsipras’s swearing-in Monday. The ratings agency Standard & Poor’s announced that it could downgrade the country before a scheduled review in March amid signs that political upheaval has already dented Greece’s embryonic recovery.

Investors have been nervous that a showdown with Europe could end badly for Greece, and on Monday there was little indication from European leaders that they would consider the sort of fundamental renegotiation that Syriza demands.

Newly sworn-in Greek Prime Minister Alexis Tsipras waves to spectators and media on his way to enter the prime minister's offices, the Maximos Mansion, in Athens on Jan. 26. (Pantelis Saitas/European Pressphoto Agency)

Britain’s chancellor of the exchequer, deficit hawk George Osborne, called Syriza’s campaign promises “very difficult to deliver and incompatible with what the euro zone currently demands of its members.”

Germany’s largest newspaper, Bild, urged the country’s leadership to hold firm in any talks. “An agreement is an agreement,” the paper declared.

But according to Syriza, the agreements are already dead. In an interview Monday evening, leading Syriza economist Yanis Varoufakis said the party considers Greece’s bailouts to be “finished” and will demand immediate debt forgiveness from Germany and others.

“Germany’s not interested in Greece. They consider us to be insufferable grasshoppers,” said Varoufakis, who is considered a likely prospect for finance minister. “We were being sacrificed on the altar, and we’re not going to allow ourselves to be sacrificed anymore.”

Tsipras had declared in a rousing victory speech Sunday evening that Syriza’s election win “ends, beyond any doubt, the vicious circle of austerity in our country.”

The comment was cheered by a crowd of thousands that included not only Greeks but leftists from across the continent who had come to celebrate what they regard as the first victory in an emerging struggle to overturn a European political establishment embodied by German Chancellor Angela Merkel.

“What Tsipras will do for Greece, we want for Italy,” said Andrea Ferroni, a 23-year-old who waved the hammer-and-sickle flag of his Italian neo-Communist party.

From Spain, Pablo Iglesias, the ponytailed, 36-year-old leader of the leftist Podemos movement, tweeted that “the Greeks finally have a Greek government, and not an envoy of Merkel.”

Podemos, which didn’t exist until a year ago, has been surging in the polls ahead of Spanish elections slated for December, making Iglesias a candidate to join Tsipras in a fraternity of radical leftist European leaders that so far has but one member.

Like Podemos, Syriza has risen fast. As desperation has grown in Greece, with a quarter of the population unemployed and a third in poverty, the party and its collection of Maoists, Marxists and Trotskyites has rocketed from the fringe to the pinnacle of politics.

Unlike the socialists who long governed Greece, Syriza proudly presents itself as a radical movement far outside the political mainstream. Tsipras until recently kept a portrait of Che Guevara on his office wall, and he got his start in politics as a youth activist for the Communists.

Unlike most top Greek politicians, Tsipras was educated in Greece, not abroad, and he still lives in a modest, rented apartment.

“He’s a comrade. I can talk to him and say, ‘Hello comrade, how are you?’ ” said Stasa Kotara, a 25-year-old Syriza activist who said she’s met Tsipras several times when he’s visited sleep-away camps for young leftists.

Tsipras is a civil engineer by training, but many of his top advisers are respected economists who have spent decades in the classroom preaching the virtues of leftist philosophy.

None, however, have significant experience in government — either in running a bureaucracy or in conducting the high-wire, international negotiations that are likely to come in the next weeks and months.

Analysts say Syriza’s leadership is deeply committed to its anti-austerity ideology and feels emboldened by the support it is getting from prominent left-leaning economists across the globe.

That, said Christian Odendahl, chief economist at the Center for European Reform, could lead the party to overplay its hand.

“My fear is they will attempt to explain to Germany and the European partners how wrong their approach is and they should see the wisdom of an alternative approach,” he said. “That almost certainly is going to fail.”

What could succeed, experts said, is a recognition from leaders in both Athens and Berlin that everyone has a lot to lose if Greece slides out of the euro, as some fear it could absent a deal. Even if the economic consequences of an exit are fairly minimal beyond Greece, the political repercussions could be huge for a continent where unity is being tested on multiple fronts.

“The mutually assured destruction is hopefully enough of a deterrent for both sides,” Malkoutzis said. “Some common ground has to be found, and pride has to be swallowed.”

Sergio Tsitakis in Athens and Ishaan Tharoor and Brian Murphy in Washington contributed to this report.