Comic Beppe Grillo, founder of the anti-euro movement, lashed out at print and TV journalists, accusing them of fabricating news to keep the Five Stars down.
"Newspapers and television news programmes are the biggest manufacturers of false news in the country, with the aim of ensuring those who have power keep it," he said on his blog on Tuesday.
He called for "a popular jury to determine the veracity of the news published," and said in cases of fake news "the editor must, head bowed, make a public apology and publish the correct version at the start of the programme or on the paper's front page".
Grillo - who has himself been accused of involvement in a network of fake news sites - said members of the general public "picked at random" would be shown newspaper articles and programmes and asked "to determine their accuracy."
The blog was accompanied by a montage of the banners and logos of Italy's main newspapers and television news programmes.
The media world was enraged. The news director of the private TG La7 channel, Enrico Mentana, said he would sue the comedian, while journalists' union FNSI slammed the "lynching of all journalists".
The opposition Five Stars was running neck-and-neck with the ruling centre-left Democratic Party (PD) before Matteo Renzi's downfall last month and is campaigning hard for the next general election, which could be held in coming months.
What Grillo is proposing "is called Fascism, and those who play it down are accomplices," PD senator Stefano Esposito said.
The centre-right Forza Italia (FI) party, founded by ex-prime minister Silvio Berlusconi, said Grillo wanted a "minculpop 2.0", a reference to the propaganda and censorship ministry under dictator Benito Mussolini.
Grillo has had a difficult relationship with the media since launching the Five Stars (M5S) in 2009, banning members from appearing on talk shows and giving international media priority over their Italian counterparts at his rallies.
His claim that journalists were to blame for the country's poor standing on the World Press Freedom Index - where it ranks 77th - was dismissed on Wednesday by the editor in chief of the Repubblica daily.
'Shady deals, big lies'
"It's not because of an enslavement to power, but the opposite: there are too many journalists threatened by the mafia and organized crime groups over their investigations into shady dealings and corruption," Mario Calabresi said.
"And if that wasn't enough, we hold the record for the number of lawsuits against journalists brought by politicians who cannot bear the idea that someone would look into their business or criticize them," he added.
Marco Travaglio, editor of the far-left Il Fatto Quotidiano daily, agreed with Grillo but said the popular jury idea would never work.
"The biggest lies are those spread by the television and newspapers, but the solution he is proposing is naive," he said.
Grillo's comments follow widespread debate in the United States and Britain over whether fake news may have played a part in persuading people to vote for Republican candidate Donald Trump and for Brexit.
Last month, Pope Francis weighed in, comparing scandal-obsessed media outlets who smear politicians and spread gossip to people sexually excited by excrement - and describing their readers as faeces eaters.
By Ella Ide