ROME — A grass-roots movement protesting the populism of the far-right Italian leader Matteo Salvini demonstrated its surging strength on Saturday, drawing tens of thousands of people to a vast square in Rome in its first national rally.

The Sardines movement, named for its ability to pack piazzas, reflects a general disgust among many liberal Italians over Mr. Salvini’s anti-migrant and anti-European language.

“Something has already changed in the Italian political panorama,” said Mattia Santori, 32, one of the movement’s founders. He said its chief purpose was to combat apathy and to offer equality, respect for the Constitution and stronger Italian institutions as a way to counter hate and Mr. Salvini’s essential themes: opposition to immigration, antagonism toward Europe and heightened security.

The Sardines were inspired to organize by Mr. Salvini’s campaign vow to “liberate” the liberal stronghold of Emilia Romagna in regional elections next month. They fear that if Mr. Salvini can win there, he can win anywhere.