Election in two weeks' time could see Beppe Grillo's Five Star movement in position to stop left or right from wielding majority

In the shadow of the Alps, at nine o'clock on a February evening, the term "warm-up act" had a literal meaning: the singer on the stage in the square was wearing an amply padded anorak.

Five minutes before Beppe Grillo arrived, the temperature had fallen almost to freezing. Yet this piazza, like every piazza he has addressed on his campaign tour, was packed to bursting.

The man who could throw Italy into turmoil after the general election on 24 and 25 February began conversationally, but was soon roaring his execration of his country's established politicians to a crowd of several thousand.

"They're terrorised," he cried. "They're frightened because this movement" – he waved a hand over the square – "is becoming something extraordinary." With a gesture, he had gathered everyone present into something he said was "no longer a movement, but a community".

After fading in the polls, Grillo's Five Star Movement (M5S) is surging back, its cause boosted by the scandal at Italy's third-biggest bank, Monte dei Paschi di Siena (MPS). Since MPS was always beholden to the left, the scandal proves to many Italians what Grillo has always claimed – that Italy's politicians are all the same and, in a phrase he used several times at his rally, should be mandati a casa – sent packing.

In response, the mainstream leaders had called him all sorts of names: "populist", "demagogue", "megalomaniac", he told the crowd before inviting them to do the same. "One … two … three …" "POP-U-LIST-A", the crowd roared back. It was a neat way of dodging the most pertinent criticism of his movement.

The next morning, stuffing down a late breakfast, red-eyed having gone to bed long after midnight, Grillo acknowledged that the classic definition of a populist was someone who denied the existence of left and right. So where did he stand on the political spectrum?

"Above it," he replied with a grin. Pressed by the Guardian, he said the M5S was "a movement of ideas, not of ideologies". His followers were "conservative revolutionaries".

Grillo's earliest political target was a system that funnels millions of Italian taxpayers' euros to parties riddled with corruption and allows men and women convicted of criminal offences to sit in parliament. Laying his knife on the tablecloth to stress the point with both hands, he said: "These are people judged guilty in [Italy's highest appeals court] who make laws."

No one with a conviction – and that includes Grillo, found guilty of manslaughter after a fatal accident – can be an M5S parliamentary candidate. Nor will his movement take public subsidies. Its regional parliamentarians in Sicily, where it topped the poll in an election last year, bank only 30% of their salaries, he said. The rest goes into micro-credit schemes for small businesses.

At the rally Grillo mocked each of Italy's party leaders in turn. He was worried the centre-left leader Pier Luigi Bersani could tear him to pieces "with his gums".

Like Silvio Berlusconi, and unlike almost every other Italian politician, Grillo understands that people who turn out for political meetings want to be amused while they are hectored. And hector them he does, bombarding them with facts and figures to illustrate the state of their country: Italy, he told his audience, had 350,000 laws, 9m court cases pending, and a third of all the lawyers in the EU.

Interlaced with the statistics and the gags is a vision of the future that chimes with the ideas of environmental activists, internet libertarians, the anti-free market campaigners of Occupy Wall Street, and the fair trade and slow food movements. But it also includes some patently conservative themes.

Grillo is restrained – almost affectionate – in his criticism of Berlusconi, and got his biggest cheer at the rally for an attack on the tax collection agency, a hate target for traditionally rightwing small-business owners and self-employed workers.

The M5S, which sprang from Grillo's hugely popular blog and coalesced on the www.meetup.com website, is often viewed as a political expression of the net generation. But the average age of the audience in Udine, in north-east Italy, seemed to be at least 40. "What surprises me is that we are seeing more middle-aged people," Grillo agreed.

A recent academic study found evidence that the M5S has been attracting growing numbers of supporters who define themselves as right of centre. That would suggest it is tapping into the pool of support pollsters say will be decisive in this election: the 15% who intend to vote but do not yet know for whom. Most are disillusioned former supporters of Berlusconi or the viscerally rightwing Northern League.

The last surveys published before a ban on polling during the runup to the election, which took effect on Saturday, gave the M5S 13% to 16%, making it Italy's third biggest party. That alone would translate into 30 seats in the 315-seat senate, the decisive battleground for the next government.

But if the M5S's support grows at the same pace as in recent weeks, it could be in a position to stop both left and right from wielding a majority – an outcome that would no doubt panic the markets and trigger a new eurozone crisis, since Grillo has ruled out any coalition with the established parties. It would also place an awesome responsibility on the M5S's new politicians. Without exception, they are political newcomers chosen by other members of the movement in an internet ballot.

"This could be a real tsunami. Those polls … we're at twice that level," said Grillo. And then, half-audibly, he added: "Unfortunately."

One of his assistants said: "You mean 'Unfortunately for the other parties'." But Grillo did not respond.