Barefoot and wearing faded jeans and a gray T-shirt with an image of Gandhi, he said his goal was to do away with a system that had “disintegrated the country” and build “something new” that would restore Italy to a true participatory democracy. “We can change everything in the hands of respectable people, but the existing political class must be expelled immediately,” he said.

Mr. Grillo typifies a new style of politician rising from the fires of the European Union’s long-running economic crisis and voter discontent in other countries. Like Alexis Tsipras, the young upstart in Greece who rode an anti-austerity wave to head the second-largest parliamentary party, or Yair Lapid, who tapped into a national frustration with social inequality in Israel, these politicians are not extremists but generally reformist leftists.

In refusing to play ball with the establishment powers, whom they consider irredeemably corrupt, the newcomers are fraying nerves in Brussels and Berlin.

“There is the sense that the European project is at risk with what happens in Italy if Grillo blames the E.U. for problems along with Germany, the enforcer,” said Moisés Naím, a senior associate at the Carnegie Endowment for International Peace in Washington. “If he gets away with that, other politicians will also do it, and there may be contagion.”

Now that his political crusade, called the Five Star Movement, has been handed a powerful mandate, Mr. Grillo’s challenge will be to decide how to use it to change an establishment that has long viewed him as a demagogic, even reckless, man who risks taking Italy down the path of Greece.