Italy plunged deeper into political chaos this weekend after Beppe Grillo, the quixotic former comedian who holds the balance of power in parliament, suggested that the country may have to abandon the euro and return to the lire.

The rebel comic's warning came amid a growing rebellion among grass-roots supporters of his Five Star Movement, with 150,000 signing a petition calling for him to open up dialogue with the centre-Left Democratic Party, the biggest force in parliament.

With the country in political paralysis, there were also questions over his eccentric behaviour, after the surreal public appearance of a man, either Mr Grillo or one of his supporters, with his face obscured by a zipped up puffer jacket and a pair of ski goggles.

The bizarre figure, resembling a human fly, waved at photographers from the deck of Mr Grillo's beach house at Marina di Bibbona on the coast of Tuscany.

In an interview with a German magazine, Mr Grillo warned that "if conditions do not change" Italy "will want" to leave the euro and return to the lire. The 64-year-old comic-turned-political activist also said Italy needs to renegotiate its €2 trillion debt.

At 127 per cent of gross domestic product (GDP), it is the highest in the euro zone after Greece. "Right now we are being crushed, not by the euro, but by our debt," he told Focus, a weekly news magazine. "When the interest payments reach €100 billion a year, we're dead. There's no alternative."

He said Italy was in such dire economic straits that "in six months, we will no longer be able to pay pensions and the wages of public employees".

The comments will further hamper efforts to resolve the crisis caused by Italy's general election last week, in which Mr Grillo's web-based, anti-establishment movement won more than a quarter of the vote .

He is refusing to contemplate any sort of power-sharing deal with the centre-Left Democratic Party, which has shaky control of the upper and lower houses of parliament but lacks a big enough majority to form a government.

In the Focus interview, he said that an accord with the big parties would in theory be possible if they acceded to key parts of his movement's agenda, including limiting MPs' parliamentary service to two terms, an overhaul of the election system and the slashing of the lavish perks enjoyed by politicians. "But they will never do that," he said. "They are trying to make us believe that they will, just to gain time."

Mr Grillo, renowned for his blistering attacks on Italy's established political caste, last week called Pier Luigi Bersani, the leader of the Democratic Party "a dead man talking".

In the interview published yesterday he predicted the annihilation of both the centre-Left and the centre-Right coalition led by Silvio Berlusconi, the 76-year-old playboy who has been prime minister three times. They would be around for another "six months" but then they would be "finished", he said.

He also insisted that his party would not take part in any "horse-trading", describing the overtures from the Left as "the usual whorish way of doing politics."

But he faces a growing clamour among his grass-roots supporters, however, to open up dialogue with the Democratic Party in order to break the log jam and form some sort of credible government.

An online petition begun on Wednesday by Viola Tesi, a 24-year-old member of the Five Star Movement from Florence, had by yesterday gathered nearly 150,000 signatures.

Miss Tesi wrote an open letter to Mr Grillo on the website, www.change.org, appealing to him not to "waste" her vote but to give a confidence vote to the Democratic Party so that a reforming government can be formed.

Grillo supporters should harness their unexpected triumph at the polls to compel the centre-Left to adopt the policies that would make Italy a better country, she said. "We should embrace the challenge and start changing Italy straight away, for the benefit of all," she wrote in the letter.

But with Mr Grillo apparently ignoring those appeals, the three-way deadlock has raised fears about Italy's ability to maintain desperately needed economic reforms initiated by Mario Monti, the former technocrat prime minister.

The comedian has said that his movement will remain outside any government, voting on individual bills in parliament on a case-by-case basis.

Adding to the uncertainty is the fact that Mr Grillo refuses to talk to Italian journalists or to appear on current affairs television talk shows.

Italian reporters who have telephoned him and asked to speak to the general secretary of the party claim he has told them: "Hang on, I'll just pass you to my 12 year old son."

The new parliament has to meet by March 15 at the latest, after which formal talks with Giorgio Napolitano, the octogenarian Italian president, are scheduled to begin on the formation of a new government.

In the space of just three years, Mr Grillo's movement has come from nowhere to place him in the role of kingmaker. During the election campaign he travelled Italy in a camper van in what he dubbed his "Tsunami Tour", filling piazzas with cheering supporters as he railed against entrenched political and business interests.

One prominent supporter is Dario Fo, the 86-year-old playwright and satirist, whose best known works include Accidental Death of an Anarchist and We Can't Pay, We Won't Pay – an apposite title given Mr Grillo's threats regarding Italy's debt.

Mr Fo said yesterday: "Satire can uncover big ideas and bring about huge upheavals. Beppe Grillo is a man of satire, that's where he came from, and that is his strength."

Mr Grillo's success means his party is poised to send more than 160 members into parliament's two chambers. Many are in their twenties and thirties and very few have any political experience, raising fears of legislative chaos if a government is eventually formed.

The "Grillini", as they are known, include nurses, teachers, students, lawyers, engineers, molecular biologists, bank clerks, and architects.

Some are unemployed. All are under 50 – a novelty in a country in which many politicians are well into their seventies.

Critics say they will be clueless in parliament and that their reliance on internet-based democracy to determine policies will be a recipe for disaster. Mr Grillo says their lack of political experience means they are untainted by corruption and cynicism.

One new MP, Ivan Catalano, 26, who works at a metal machining factory north of Milan, told The Sunday Telegraph: "I am a normal citizen like any, who thanks to this new way of doing politics, will enter into parliament.

"The message is that change is possible. We don't have to put up with the same old methods, and the same old people."

He acknowledged there were differences of opinion within the movement but denied it was an open schism. "It is a healthy dialogue," he said.

The movement's policies include tax cuts, a big increase in health spending and investment in the "green economy", but it has struggled to explain how this will be paid given Italy's mountain of debt.

Mr Grillo also wants deep cuts in defence spending and the scrapping of a high-speed rail link to France beneath the Alps.

The movement's newly-elected MPs and senators will meet one other for the first time in Rome on Monday to discuss whether to ignore their leader and support some sort of coalition.

Italians are divided as to whether the movement promises fresh blood for an anaemic, corrupt system, or political and economic disaster for a country already mired in recession.

"Beppe Grillo is a pistol pointed against Italy's head," was the front cover headline in last week's edition of Panorama, a conservative news magazine. Another news weekly, Oggi, ran a photograph of Mr Grillo with the words "Now what?"