A grassroots movement which emerged in Bologna last week to oppose Italy's far-right League party leader Matteo Salvini is now spreading across the country, gaining thousands of followers.

The movement, known as "sardines", started off as the idea of four friends. Fed up with Salvini's rhetoric, they decided to call on people to join protests against the former Italian interior minister, who was campaigning in Bologna ahead of the January 2020 elections.

More than 15,000 people gathered in Bologna's Piazza Maggiore, "packed in like sardines" under the pouring rain, outnumbering Salvini's own rally which gathered a much smaller crowd of 3,000.

The success of the first demonstration prompted another protest on Monday in the city of Modena, where an estimated 7,000 gathered to protest against the politics of Salvini. The square was filled with generations of Italians singing the anti-fascist resistance song Bella Ciao and repeating the slogan "Modena non si lega" - "this city does not show allegiance to the League."

Videos and images of the protests were shared on social media and the public response has been overwhelming, with people asking to organize more events all across the country.

On Friday 6,000 "sardines" filled the historical Piazza Verdi in Palermo, Sicily, and more flash mobs are planned in 10 other Italian cities between now and December. A national-level demonstration is planned for 14 December in Rome, where organizers hope to gather up to a million people.

The four friends and founders of the movement, Mattia, Roberto, Giulia and Andrea, have launched a Facebook page which already has 131,000 followers. They have also published the movement's manifesto, which reads like an open letter to politicians. The opening lines say "Dear populists, you have now realised it. The party's over."

The "sardines" call themselves "normal people who love their families and their homes" and still believe in politics and politicians "with the capital P," the kind of politicians "who fail but at least try, who think about their personal interest only after thinking about the public one."

"We are already hundreds of thousands and we are ready to tell you to stop," declares the manifesto, which threatens to "spread this message until you'll feel seasick. We're free sardines and you'll find us everywhere now. Welcome to the open sea." 

Salvini has accused the centre-left Democratic Party (PD) of hiding behind the movement, but although the PD's Nicola Zingaretti has expressed support and even gratitude towards the movement, the "sardines" claims to be a non-partisan initiative: participants are asked to leave flags and political affiliations at home. The four friends at the origin of the movement are not politicians but 30-year-olds with normal jobs (one is a frisbee instructor, one a tourist guide, one a physiotherapist, and one an engineer). 

In response to the protests, Salvini has called upon his social media followers to defend him online and has tried to ridicule and minimize the impact of the movement, first thanking them for the extra publicity they are giving him and finally announcing on Twitter that he prefers "kitten to sardines, and kittens eat sardines when they're hungry."

According to a survey conducted by Index Research for the Italian TV show Piazzapulita, 43.6 percent of Italians think that the "sardines" now represents "Salvini's most dangerous enemy", a result that certainly isn't flattering for the Democratic Party, which was only seen by 14.4 percent as capable of opposing the rise of the League.

Eight Italian regions will vote in the 2020 elections, and polls suggest the rise of the League - and the centre-right alliance of Silvio Berlusconi and Giorgia Meloni - could lead the left-wing parties to lose regions they have traditionally held for decades.

The "sardines" movement is now shaking up the political debate in Italy and has promised months of demonstrations against Salvini leading up to the elections.