The anti-establishment 5-Star Movement's leader Beppe Grillo called on Italians to join him in protest outside parliament after President Giorgio Napolitano stood for a second term on Saturday in what Grillo called a "coup d'etat".
Grillo, who drew hundreds of thousands to a rally in Rome before a February election in which his party of political newcomers claimed one in four votes, declared he was immediately abandoning a campaign in the north of Italy to drive 650 km (410 miles) to the Rome parliament.
"There are decisive moments in the history of a nation," the former comedian wrote in a blog post titled 'call to Italians'. "Tonight I will be in front of parliament. I will stay there as long as is necessary. There have to be millions of us."
Grillo says he is convinced traditional parties he blames for Italy's economic decline and corruption have already agreed to govern together in a coalition to preserve the status quo.
He described an agreement between leaders of the main center-left, center-right and centrist parties to ask Napolitano to run again to break a political deadlock as a "coup d'etat".
His words were condemned by mainstream politicians, some of whom said his language and planned protest were reminiscent of wartime dictator Benito Mussolini's "march on Rome" which marked his rise to power in 1922.
The area around parliament has been blocked off by police barriers and fences since voting began on Thursday, but daily protests have taken place in a square across from the building, with demonstrator heckling deputies as they enter to cast votes.
On Saturday, protesters chanted in favor of the presidential candidate chosen by 5-Star supporters in an online vote, the left-wing academic Stefano Rodota, and held up banners reading "Italy screams for Rodota as president".
The 5-Star Movement rode a huge protest vote to become one of three main forces in parliament, driven by frustration at economic hardship and a discredited political class.