Italians go to the polls amid market jitters over the power of the comic who would cut top pay, offer a vote on euro membership and rip open politics 'like a can of tuna'

As an enormous crowd gathered to hear him speak outside Rome's San Giovanni basilica on Friday night, the comedian Beppe Grillo had every reason to be jubilant, but the thought of bringing down Italy's political order appeared to make him pensive, even melancholy.

"It's been like this up-and-down Italy – we are witnessing a change in civilisation, not just politics," he told the Observer. "Italy has turned its back on its political class and a new language of community, identity and honesty is filling the gap," he added slowly.

Minutes later, a different, wilder Grillo took the stage to a rock-star welcome before at least 100,000 cheering fans, yelling at them that his movement would rip open parliament "like a tin of tuna" when it sends an army of activists – analysts predict more than 100 – into the senate and lower house after Italians go the polls today and tomorrow.

Turning his attention to Italy's pampered political caste, he screamed "It's finished! Give up! You are surrounded!" as fans waved a banner stating, "We want to get out of the darkness."

Friday's massive rally was the climax to a marathon tour of packed piazzas which has pushed Grillo's poll ratings towards 20%, panicking pundits who had predicted a market-pleasing outcome to the election in which the centre-left leader, Pier Luigi Bersani, would form a coalition with outgoing technocrat prime minister Mario Monti.

Silvio Berlusconi, hitherto seen as the potential spanner in the works, slipped back in the final days of campaigning, drawing flak for making lewd remarks to a woman during a speech and condoning bribery in a TV interview, while his promise to pay back an unpopular tax left many unconvinced.

It is Grillo, not Berlusconi, who has surfaced as the new bogeyman for the markets, his popularity threatening gridlock in parliament or at best a weak centre-left coalition which will collapse within months. Grillo, many argue, may be a well intentioned experiment in populist "anti-politics" that ends up dragging Europe back into a debt crisis.

His manifesto combines pro-environment policies with a crackdown on parliamentary privileges, a living wage for the jobless using cuts from military spending, the slashing of top managers' wages, broadband for all, bike lanes and the right for priests to have children "so they don't touch other people's".

But what has spooked the markets is his commitment to hold a referendum on leaving the euro and a temporary freeze on interest payments on government bonds, which could lead to default. Meanwhile, Grillo's decision not to talk to the Italian press, which has led to accusations that he is ducking tough questions, reached a climax on Friday when Italian journalists were kept from going backstage at the Rome rally while foreign reporters were let in, prompting police to open the barriers after a furious row.

"With him we'll be worse off than Greece – we will say goodbye to democracy," warned Bersani.

As Italy sinks into its longest recession in 20 years, thanks to almost negligible growth and a two trillion euro debt pile, political stalemate is the last thing the country needs. But on Friday, Grillo was in no mood to cut deals, working himself into such a frenzy that even the face of the woman on stage beside him signing for the deaf looked furious.

The 64-year-old comic will not enter parliament – he has a 1980 driving conviction for manslaughter after a crash in which passengers were killed, and thus falls foul of his own rule banning MPs with records.

Slowly, his inexperienced parliamentary candidates have been emerging from his shadow, and on Friday, some were suggesting that some of Grillo's wilder plans, such as freezing the debt, could be taken with a pinch of salt. "Our ideas are a work in progress, Grillo will never give us orders and never has," said Giuliana di Pillo, 50, a teacher of disabled children and senate candidate who said she would fight for more school funding. Di Pillo's last visit to the senate chamber was 10 years ago, when she took a group of schoolchildren: "I never once imagined I would return as a senator."

Milling around backstage at the Rome rally, Di Pillo chatted to other candidates whose measured speech contrasted with Grillo's stage roar.

"Grillo's not the boss – he is there to check the right people join the movement, but the responsibility to administrate will be ours, as citizens," said Alessandro di Nicola, 33, a construction worker standing for regional councillor in Rome.

Laura Pizzotti, 52, an IT specialist who is standing for the senate, said the so-called "Grillini" were open to teaming with other parties in parliament on bills they approved of. "We will be there for the good of Italy and that means we won't try to bring parliament to a halt," she said.

Pizzotti joined Grillo's blog-based movement in its early days, seven years ago, and has fought to keep Italy's water supply out of private hands, a common cause among Grillo's supporters. Now she wants to stifle funding for political parties. "We will be at sea at the start, but we are studying and there will be experts in parliament to help us write laws," she said.

"We have a network of architects, engineers and legal experts ready to give back-up," said Di Nicola.The candidates – a far cry from the left-wing extremist label given them by Berlusconi – described Grillo as their "megaphone", whose internet-fuelled rage appealed to Italians sick of being told by slick politicians that nothing will change. "Grillo is a reaction to bad politics, but whatever happens in parliament will be weighed on the scales with the citizens," said Pizzotti.

Grillo backers claim they are already making a difference after taking over the town hall in the city of Parma and becoming the biggest party in Sicily in regional elections last year.

Once in office in Sicily, Grillini cut bloated council wages from €15,000 (£13,000) to €2,500 and plan to use the excess to find microcredit schemes.

"When we cut the wages back, councillors from other parties did not show up for work for two days for fear of being asked by journalists why they weren't doing the same," said councillor Giancarlo Cancelleri, 37, a former office worker.

On Friday, Grillo got some of his biggest cheers from the crowd when he savaged Italy's pampered, and often corrupt, political class. "They must all go..." he said, before the crowd chanted back "Home!" As it gets ready to vote, Italy is about to find out just how radical Beppe Grillo really wants to get when he and his backers move from online forums and piazzas to the corridors of power.