Tens of thousands of people joined a demonstration in central Rome on Saturday organized by a grassroots movement known as "the sardines", launched only a month ago to protest against far-right leader Matteo Salvini.

The sardines began in the city of Bologna in November when Mattia Santori, 32, and three friends invited people to protest against Salvini's League, whose popularity is high ahead of an election in the northern Emilia-Romagna region.

Salvini hopes to lead a resurgent right to its first poll victory in the wealthy region on January 26. He says that if the left loses power there, it should also quit the coalition government in Rome and open the way for a national ballot.

Students, pensioners and families with children filled Rome's Piazza San Giovanni on a sunny afternoon, some holding pictures of sardines on cardboard and chanting a popular resistance song.

"I'm here because I'm against Salvini and I'm against the right and I want to see how many people feel like me," said Flavia Simula, a 68-year old retired biologist.

Since their maiden protest in Bologna gathered an estimated 15,000 people, the popularity of the "sardines" has spread.

They have held bigger rallies in several Italian cities, including Turin and Milan, to denounce Salvini, whose anti-immigrant, Italy-first rhetoric has resonated with many voters.

The demonstration in Rome appeared to be their largest so far, with an estimated turnout of 40,000 people.

"Our aim was to fill the square and we managed to do that," Santori said. "The sardines are just people who fill a space with their ideas and who fight an enemy - the simplification of the issues by populists."

Pollsters say Emilia-Romagna, a leftist stronghold, could fall to the League and its rightist allies following successes in other regional votes, most recently in central Umbria.

They say the right is also likely to win any national vote, with the League seen as Italy's most popular party.

"I am here to say I am against fascism, I consider Salvini a fascist," said 18-year old student Luca Baironi.

Salvini has accused the center-left Democratic Party (PD) of being behind the movement and dismissed the protests, saying he likes kittens because "they eat sardines".

The sardines deny any political affiliation and say they just want to register their opposition to populist forces gaining strength after a decade of economic stagnation.