Standing on the podium in his trademark open-neck shirt and dark suit, Alexis Tsipras clenched his fist and bellowed at the crowd.

"For two years they have taken decisions without asking us," the leftist leader boomed. "The Greek people didn't give them the mandate to take those decisions. In the birthplace of democracy, there is no democracy. The time has come to return democracy to the place where it was born."

Amid a sea of flags and horns and anti-austerity slogans, the Athens crowd roared its approval.

On Sunday, when the country holds general elections, Tsipras says Greeks will send "a message", not only to their own political elite but to "the peoples and governments of Europe". "Merkel should worry and Europe should hope in us," he told the Observer. "If the politics of austerity continue, Europe is in big danger of breaking up. These policies are causing unhappiness, unemployment and poverty, as in the 1930s. Europe needs social solidarity and not to work according to market laws."

Many Greeks, it seems, would agree. In the countdown to the ballot, no other party appears as poised for success as Syriza, the array of radical left and green groups lead by Tsipras.

In recent days, pollsters have spoken, almost breathlessly, of a spectacular rise in the leftists' ratings. Even before a ban on surveys was enforced two weeks ago, Syriza was on course to double the 4.6% of the vote it had won at the last general elections in 2009. On Friday, analysts conducting private polls spoke of Syriza capturing as much as 15% of the vote and possibly even emerging as the second largest party ahead of Pasok, Greece's mainstream socialist camp.

Ratings soared after Tsipras mooted the idea of a government of leftist forces, including the KKE communist party, which promptly rebuffed the overture. "I stand by my proposal. If we do well I will suggest that a leftist government is formed," said the politician.

Syriza makes no secret of the fact that it is counting on anger – and fear. More than two years after Europe's debt crisis erupted in Athens, Greeks want to "punish" a political establishment widely associated with the country's near economic collapse.

Charismatic and fresh-faced, Tsipras, Greece's youngest political leader, has capitalized on fears that in its "post-bailout" era, dependent on rescue funds from the EU and IMF, Greece is being turned into a "protectorate," stripped of any say in the running of its own affairs. "We want the loan agreement to be annulled," he insisted, calling the terms attached to the aid propping up the Greek economy "inhumane". "Europe is desperately in need of a Roosevelt-style New Deal."

It is a message that resonates. Backing for New Democracy and Pasok, the two main parties which have alternated in power for the past four decades, has dropped precipitously. The anti-austerity bloc could garner up to 60% of the vote. That would be enough to ensure that even if New Democracy and Pasok were able to renew the coalition that has steered Greece for the past six months under unelected technocrat Lucas Papademos, the new government would face formidable opposition.

"We are not against the euro but we are opposed to the policies being pursued in the name of the euro," said Tsipras. "Syriza can be the catalyst for great change."