ROME — “Jackal!” the activists shouted as Matteo Salvini, the strongman of Italy’s populist government, arrived at the other end of the street. “Jackal!”

Mr. Salvini, the leader of the anti-migrant League party and the country’s tough interior minister, had arrived to leave a rose at the entrance of a drug den where the police had discovered the body of a 16-year-old girl, Desirée Mariottini, who they believe had been repeatedly raped, perhaps while unconscious, and left for dead by African migrants.

But the activists in San Lorenzo, a traditionally left-wing neighborhood still scarred from the bombings of the Second World War and now bearing the fresh wounds of drug dealers and lawlessness, refused to let Mr. Salvini pass.

They carried banners reading: “Salvini exploits tragedies. San Lorenzo is not your campaign’s catwalk,” and shouted for him to leave.