Greece’s anti-austerity party of the left, Syriza, has stretched its election lead to six points, putting it on course for a historic victory in Sunday’s crucial elections.

With the incumbent prime minister, Antonis Samaras, warning of economic catastrophe if Syriza prevails, and Europe looking on nervously, the shortest election campaign in Greek postwar history concludes on Friday.

Barely four weeks after the failure of parliament to elect a president, triggering the ballot, Greece’s fate now lies in the hands of 9.8 million voters. All the polls show, with growing conviction, that victory will go to Syriza. A poll released by GPO for Mega TV late on Thursday gave the far leftists a six-percentage-point lead over Samaras’s centre-right New Democracy, the dominant force in a coalition government that has held power since June 2012. A week earlier, GPO had the lead at four percentage points.

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Buoyed by such figures, Alexis Tsipras, the young firebrand who has overseen Syriza’s meteoric rise from the margins of Greek political life, pledged “historic change” as he gave a triumphant speech to thousands of supporters in central Athens on Thursday night.

“History is knocking at our door,” he said, appealing to Greeks, young and old, to participate in the “overthrow” of an establishment widely blamed for bringing the bailed-out nation to the point of economic and social collapse. “Hope isn’t coming. It has arrived. Nothing can stop it now,” he said, attacking Samaras as “a merchant of fear”.

Few election campaigns have been as bitter or polarised as this, with Greeks divided between left and right, old and new, pro-and anti-bailout. Antipathy between Samaras, the 63-year-old scion of a family that made a fortune in the cotton business, and Tsipras, 40, is almost palpable.

After criss-crossing the country in what has been a frantic bid to win over an increasingly disgruntled populace, Samaras warned that the radical leftists would lead Greece to “utter devastation”.

Facebook Twitter Pinterest People walk past an election poster of Greek prime minister Antonis Samaras in Athens. Photograph: Orestis Panagiotou/EPA

Syriza has threatened to “cancel austerity” and stop interest payments on Athens’ monumental debt – moves that will almost certainly put it on a collision course with the international creditors that have injected €240bn into Greece since its brush with bankruptcy five years ago.

“Syriza is not a European party,” the outgoing prime minister told Ant 1 television station on Friday. “People must decide between truth and lies.”

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Analysts maintain that Syriza’s ability to attain an outright majority will be difficult. With pressure mounting from the EU and IMF to “respect” the commitments made as the price of aid, speculation has been rife that the party might prefer to enter a coalition government that would enable it to forge ahead with the structural reforms and budget cuts demanded in exchange for the biggest financial assistance programme in global history.

But Tsipras put paid to that. The leftists, who have never held office in the near 200 years of the Modern Greek state – and who, after a bloody civil war, were hounded and imprisoned for decades – wanted to win an absolute majority that would allow them to govern unimpeded, he insisted.

“We are asking for a clear mandate, crystal clear, undiluted, indisputable,” he told the crowd. “The time of the left has come.”

Dr Eleni Panagiotarea, a research fellow at Greece’s leading thinktank Eliamep, said Syriza was on a roll.

“It’s now all about making a clean break with the past. The party has picked up on the fatigue that people feel with the country. It has become a voice for the disgruntled middle class, unemployed, socially vulnerable, all those who want change.”

Facebook Twitter Pinterest Syriza leader Alexis Tsipras (L) addresses party supporters with Spanish Podemos party chief Pablo Iglesias at the Athens rally. Photograph: Yannis Behrakis/Reuters

If there was any question about whether the anti-establishment rebels had ambitions of plotting a similar course elsewhere in Europe, it was firmly dispelled when Tsipras was joined on the podium by Pablo Iglesias, the leader of Spain’s anti-austerity Podemos movement. To the strains of Leonard Cohen’s First We Take Manhattan, Then We Take Berlin, the duo punched the air and Tsipras, putting his arm around Iglesias, announced that the anti-austerians were poised to challenge the old order across the continent.