Italy's populist opposition 5-Star Movement says it is ready to run the country after voters triggered the demise of prime minister Matteo Renzi with a stinging defeat on his reforms referendum.
The ballot outcome also energised the anti-immigrant Northern League party, an ally of French far-right leader Marine Le Pen, a candidate in France's presidential race.
The blow to Mr Renzi also delivered a rebuke to Italy's industrialists, banks and other establishment institutions, which had staunchly backed the referendum.
The political upset, which could spook investors, comes just as the government had made inroads in cutting the staggering rate of youth employment and while Italy's banks have urgent need for recapitalisation.
During the campaign, the risk of political instability in Italy, Europe's fourth-largest economy, triggered market reaction, with bank stocks sinking and borrowing costs on sovereign debt rising.
The Movement, led by anti-euro comic Beppe Grillo, spearheaded the No camp on the constitutional reforms, a package aimed at updating Italy's post-war constitution that Mr Renzi had depicted as vital to modernising Italy and reviving its economy.
Characteristically confident - detractors say arrogant - Mr Renzi, 41, Italy's youngest premier, had bet his political future, or at least his current premiership, on a Yes vote win and campaigned hard for a victory in recent weeks to confound opinion polls indicating that it would probably end in defeat.
With votes counted from nearly all the polling stations in Sunday's referendum, the No's were leading Yes votes by a six-to-four margin, Interior Ministry data indicated. The turnout of 67% was especially high for a referendum, and more in line for a general election vote.
"I lost and the post that gets eliminated is mine," Mr Renzi said. "The government's experience is over and in the afternoon I'll go to the Quirinal Hill to hand in my resignation" (to President Sergio Mattarella).
Leaders of the 5-Star Movement joined the chorus among opposition forces for early elections. The 5-Stars are the chief rivals of Mr Renzi's Democrats and are anxious to achieve national power for the first time.
"Today the caste in power lost," said a 5-Star leader, Luigi Di Maio.
It was a sharp retort to Mr Renzi's characterising the reforms as an opportunity to shrink the "caste" of elite, perk-enjoying politicians by reducing the numbers and powers of senators.
"Arrogance lost, from which we'll learn many things in forming our team for government and our platform," Mr Di Maio said. "Starting tomorrow we'll be working on a government of the 5-Stars, we'll involve the energies and the free persons who want to participate."
The 5-Stars' constituency is largely internet based and bills itself as anti-establishment.
"The man alone at the command doesn't exist anymore, but the citizens who govern the institutions" do, Mr Di Maio said, minutes after Mr Renzi conceded.
Mr Mattarella, as head of state, would have to decide whether to accept Mr Renzi's resignation.
Mr Renzi is widely expected to be asked to stay on at least until a budget bill can be passed later this month and to shepherd the process of reforming electoral rules, which now as they stand, would grant the biggest vote-getting a generous bonus of seats in parliament.
With the momentum on their side, the 5-Stars expressed minimal interest in any ambitious reform.
Mr Renzi's Democrats and the centre-right opposition of former premier Silvio Berlusconi want the law changed to avoid such a big bonus to a winner, in case the 5-Stars garner the most votes.
Elections are due in spring 2018, but with Mr Renzi's stepping down, they are now likely be held some time in 2017.
Another opposition leader, Matteo Salvini, of the anti-immigrant Northern League, hailed the referendum as a "victory of the people against the strong powers of three-quarters of the world" and urged immediate elections.
"We are ready to vote as soon as possible," Mr Salvini told reporters.
Many had read the referendum as an outlet for growing anti-establishment, populist sentiment in Europe.
When Mr Renzi late last year promised to resign if the referendum was defeated, it was months before David Cameron had made his ill-fated bet that a Brexit referendum would cement the UK's membership in the European Union.
In Italy, the referendum was required because the reforms were approved by less than two-thirds of parliament. But Mr Renzi raised the stakes, turning the referendum into a virtual plebiscite on himself, when he pledged to quit if Italians turned their back on reforms to streamline the Senate and give the central government more powers at the cost of the regions.
Mr Renzi had been hoping to survive the rising populist forces that have gained traction across Europe.
Earlier on Sunday, in Austria's presidential run-off, left-leaning candidate Alexander Van der Bellen prevailed over a far-right populist.
In Italy, Mr Renzi's opponents counted on tapping into the populist sentiment rising in much of Europe as well as the US presidential victory last month by billionaire political outsider Donald Trump.