Italy’s far-right interior minister, Matteo Salvini, is suing the country’s celebrated anti-mafia author Roberto Saviano for defamation over a tweet that described him as “minister of the underworld”.

“I filed a lawsuit against Saviano, as promised,” Salvini, who is also deputy prime minister and leader of the League party, wrote on Twitter. “I accept any criticisms, but I do not allow anyone to say that I help the mafia.”

Saviano, a staunch critic of Salvini, posted the tweet in June. He targeted Salvini this week in another tweet showing the bodies of a woman and child floating in the Mediterranean and asking “how much pleasure” the interior minister derived from seeing the image. “The hatred you have sown will overthrow you,” wrote Saviano.

Salvini prompted an international outcry last month when he blocked a migrant rescue ship carrying 629 people from docking at Italy’s ports. The Aquarius, which had been on its way to Sicily, got caught in a standoff between Italy and Malta after both countries refused to allow it to dock. A week earlier, Salvini had called for an end to Sicily being Europe’s “refugee camp”.

Writing in the Guardian in response to the incident, Saviano said Italy’s war on migrants had made him fear for the future of his country. “Italians are going backwards, socially, amid an upsurge of nationalism that displays racist animus against anything perceived to be an alien body,” he wrote. “It is Italy’s duty to battle for change for the better, not to descend into the most boorish nativism. Human lives are at stake.”

Saviano, who has lived under armed guard since publishing his mafia bestseller Gomorrah in 2006, responded to the announcement of legal action by accusing Salvini of being “afraid of critical voices”.

Last month, Salvini threatened to remove the author’s police protection after a stream of criticism against his hardline migration policies.

The latest lawsuit comes a few days after a man accused of defamation against Salvini went on trial in Milan. Salvini is seeking €20,000 (nearly £18,000) in damages from the man, who in 2016 called on him in a Facebook post to “put a gun in your mouth”.