ROME — A month ago, four friends put out a call on social media for a protest against far-right populism in their city of Bologna. They were hoping the event might draw 6,000 people. Instead, it drew twice that, with everybody packed into the city's central piazza like sardines.

The Sardines — now the movement’s proper name — have become a full-fledged Italian phenomenon, drawing major crowds from north to south that most of the nation’s political parties would have no chance to match.

But while the Sardines have harnessed a measure of the grass-roots frustration common in so many global political protests this year, they are alone in one respect: The target of their ire is not the people technically in power, but rather the opposition.

The emergence of such a group would be possible only in the off-kilter world of Italian politics, where two languishing parties are running the country while holding little claim to the nation’s political energy.

Instead, it is the country’s most popular party — Matteo Salvini’s far-right League — that remains the center of Italy’s political galaxy. Italians view the League as the government-in-waiting, with Salvini as the likeliest future prime minister.

And while the success of Salvini’s anti-immigration and anti-Muslim ideas has alarmed many liberal Italians, the country’s left-leaning political establishment, like so many other mainstream parties across the world, has struggled to counter the emotion of populism as it plays out at rallies or on social media

“[The Sardines] have rapidly grown to occupy an important place on a political scene that is otherwise desolate,” one of Italy’s major newspapers, the Corriere della Sera, said in an editorial.

The Sardines have broken through, in part, because they so clearly stand for one thing: opposing Salvini. They are not a political party — not yet, at least. They have become, instead, a gathering point for people who were turned off by politics, even as they worried about how a Trump-like politician might be remaking the country.

Since starting in Bologna, the Sardines have held nearly 100 events across Italy, packing Rome, Turin, Milan and other cities with crowds toting all things sardine — hand-drawn sardines, cutout sardines painted with the European Union flag, E.U. flags decorated with sardines, signs shaped like sardines denouncing fascism and hate.

“How many times did you feel a stomachache while reading the comments under Leaguer posts?” the Sardines’ official manifesto says. “How many times did you tell yourself, it can’t be true? Well, the time has come to change the inertia of populist rhetoric.”

In an interview, Mattia Santori, 32, one of the group’s founders, said the group’s very first event “lifted the lid off a pressure cooker that had been boiling for a long time” and gave people an outlet to push back against what he called Italy’s drift to the “xenophobic right.”

“[Salvini] was filling squares, newspapers, social networks,” Santori said. “We’ve entirely destroyed the perception of those who saw him as Italy’s sole master.”

Until August, Salvini had been the country’s interior minister and, unofficially, its nativist face. He made it harder for refugees to gain asylum and closed Italian borders to new migrants while taking jabs at European bureaucrats and liberal “do-gooders.” He dominated a two-party coalition with the anti-establishment Five Star Movement. Then, in a bid for more power, he tried to provoke a crisis and force a new election. Instead, Five Star made a backroom deal with its longtime enemy, the center-left Democratic Party.

Salvini was locked out of power, at least temporarily, but he has used the time on the sidelines to weaken the government. He has ripped Prime Minister Giuseppe Conte for policy decisions, and in one episode won agreement for his criticisms from Five Star leader Luigi Di Maio.

Meanwhile, the marriage between Five Star and the Democratic Party has done neither side much good. Both parties have slipped in popularity in recent months, and they were shellacked in October elections in Umbria, where the center-left lost power for the first time in decades. The League won 37 percent of all votes. Five Star, at such a low point, had been debating whether to even field a candidate.

Five Star, of course, is a reminder of how Italian politics can be topsy-turvy — and how movements can fizzle out with little warning. Several years ago, it was the Five Star Movement that successfully tapped into grass-roots economic angst and grew in popularity as an opposition party, only to disappoint after entering the government and fail to fulfill promises. Some pundits have speculated that the Sardines could lose traction if they try to more formally draw up policies, or as they become a more official movement.

“They fill the squares in the name of a very generic platform,” said Massimiliano Panarari, who has written a book on the Five Star Movement. “But there is a clear demand for an innovative political organization on the left.”

Though the Sardines have exposed the weaknesses of the Italian left, they are also helping it. The movement is supporting a Democratic incumbent in Emilia-Romagna, an area that amounts to Italy’s liberal heartland. With regional elections one month away, polls show that the far-right League is within striking distance. It was Salvini’s pledge to conquer that region, which includes Bologna, that helped inspire the Sardines’ formation.

“Should the center-left lose, it would be an avalanche, and we’d have snap elections,” said Ilvo Diamanti, a professor of political science at universities in Paris and Urbino, Italy. “It’s no coincidence that Sardines were born there, spread from there.”

Salvini has taken notice of the new form of opposition.

Last month, he asked on Twitter, “What is cuter and sweeter than kittens?”

The post depicted an altered version of the League party’s logo. It showed a photo of a kitten chomping on a sardine.