Trump set off the furore by calling Leo "terrible," drawing a rare direct response from the Pope, who said he would continue speaking out against the US-led war on Iran and in defence of migrants.
Meloni issued an initial statement backing Leo as he flew off on an ambitious four-country visit to Africa but made no specific mention of Trump's broadside.
Opposition politicians accused her of lacking the courage to directly challenge Trump, prompting her to issue a second statement later in the day to clarify her position.
"I find President Trump's words towards the Holy Father unacceptable. The Pope is the head of the Catholic Church, and it is right and normal for him to call for peace and to condemn every form of war," she said.
Meloni was the only European leader to attend Trump's inauguration in 2025 and she had hoped their friendship would boost her standing at home and abroad.
However, Trump risks becoming a liability, with 66 per cent of Italians having a negative view of the US leader.
Deputy Prime Minister Matteo Salvini, who has also in the past associated himself closely with Trump, also distanced himself from the US leader on Monday.
"Pope Leo is a spiritual leader for billions of Catholics, but beyond that, if there is one person striving for peace, it is Pope Leo, and so attacking him does not seem either wise or helpful," he said in a statement.
The Pope is the bishop of Rome and spiritual leader to millions of Italian Catholics, making politicians of all stripes wary about taking him on.
"It has been centuries since such a blatant act of aggression against the Roman pontiff was seen," former progressive prime minister Matteo Renzi said, adding that it was vital for Catholics and non-believers alike to defend Leo.
"He is, after all, a 'builder of bridges,' unlike Trump, a destroyer of relationships and of civilisation. The only advantage is this: Trumps come and go, popes remain," he said.
The comment echoed an Italian saying, "chi mangia papa crepa" which roughly means, "whoever tries to devour the Pope dies" - a proverb born of centuries of tension between successive papacies and temporal rulers.
"Trump has made the mistake of the century, because 'chi mangia papa crepa' has been borne out repeatedly," church historian Alberto Melloni said, pointing to Italy's royal family, the House of Savoy, which clashed repeatedly with the Vatican during the 19th century only to be swept away while the papacy lived on.
Antonio Spadaro, a Roman Catholic priest and undersecretary of the Vatican's Dicastery for Culture and Education, said Trump's online post revealed his own weakness.
"If Leo were irrelevant, he would not merit any comment. Instead, he is invoked, named, opposed - a sign that his words matter," Spadaro wrote on X.
"This is where the Church's moral force emerges. Not as a counter-power, but as a space in which power is judged by a standard it does not control."
Meanwhile, Trump on Monday sought to play down backlash over a self-portrait depicting him as Jesus, saying he did not view the image in religious terms.
The image, which appeared to be AI-generated in the style of a biblical painting, was published on Trump's Truth Social account amid the row with the Pope.
It showed Trump wearing a long white robe and placing a hand on the forehead of a man lying in a hospital bed.
An orb glowed in one of the president's hands while beams of light emanated from the man's head.
The post, along with the image, was later deleted after drawing angry reactions from conservative Christians, many of whom denounced it as blasphemous and narcissistic.
Trump acknowledged posting the image but said the furore stemmed from a misunderstanding.
"I thought it was me as a doctor, and had to do with Red Cross, as a Red Cross worker, which we support," Trump said.
with DPA
