President Donald Trump has met with Pope Francis, the famously humble pontiff with whom he has publicly clashed, concluding his tour of the ancestral homes of the world's three largest monotheistic religions.

Trump, midway through his nine-day maiden international journey, called upon the pontiff at the Vatican tonight NZT where the two had a private 30-minute meeting laden with religious symbolism and ancient protocol.

The president, accompanied by his wife and several aides, arrived at the Vatican just after 8am local time. The president greeted Francis in Sala del Tronetto, the room of the little throne, on the second floor of Apostolic Palace Wednesday morning.

Read more: The key issues Donald Trump and Pope Francis disagree on

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The meeting between Donald Trump and Pope Francis could provide powerful imagery to Catholic voters back in the United States. Photo / AP
The meeting between Donald Trump and Pope Francis could provide powerful imagery to Catholic voters back in the United States. Photo / AP
Donald Trump met the pope on his first foreign tour as President. Photo / AP
Donald Trump met the pope on his first foreign tour as President. Photo / AP

"A person who thinks only about building walls, wherever they may be, and not building bridges, is not Christian," Francis said then.

The pontiff has been a vocal advocate for aiding refugees, particularly those fleeing the violence in Syria, deeming it both a "moral imperative" and "Christian duty" to help.

Trump has never been one to let an insult, perceived or real, go by without a response, and he made no exception for the world's best-known religious leader. He called Francis "disgraceful" for doubting his faith.

And even the pontiff's congratulatory message sent to mark Trump's inauguration contained a sly reference to their disagreement, as the pope wrote that he hoped the United States' international stature would "continue to be measured above all by its concern for the poor, the outcast and those in need."

Trump arrived in Rome Tuesday evening, his motorcade closing a busy Italian highway just after rush hour and prompting hundreds of onlookers to briefly step out of their gridlocked cars to gawk at the fleet of armored vehicles.

Pope Francis and Donald Trump had a private meeting as part of the visit. Photo / AP
Pope Francis and Donald Trump had a private meeting as part of the visit. Photo / AP

He spent the night at the US ambassador to Italy's residence.

Though both Trump and Francis are known for their unpredictability, papal visits with heads of state are carefully arranged bits of political and religious theatre that follow a specific programme, with little room for deviation or unwanted surprises.

Trump was expected to be given a tour of the Vatican after he arrived and then meet with the pontiff in his library.

The two men were to be left alone with a translator to hold a private discussion before emerging again to exchange gifts and farewells.

A person who thinks only about building walls, wherever they may be, and not building bridges, is not Christian.

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Pope Francis and Donald Trump have clashed over their differing views. Photo / AP
Pope Francis and Donald Trump have clashed over their differing views. Photo / AP

Trump is the 13th president to visit the Vatican and, as part of his tour, he will view the Sistine Chapel.

In recent days, Francis and Trump have been in agreement on a need for Muslim leaders to do more against extremists in their own communities. But there are few other areas where their views align.

The president's prior anti-Muslim rhetoric - including his musing that Islam "hates" the West - is the antithesis of what the pope has been preaching about a need for dialogue with Muslims.

Francis also differs sharply with Trump on the need to combat climate change and economic inequality.

And he could react to events this week, including the release of Trump's budget, which would dramatically cut funding to programmes that help the poor, and the president's agreement to sell military equipment to Saudi Arabia.

Melania Trump was among the members of Donald Trump's family who also met with the Pope. Photo / AP
Melania Trump was among the members of Donald Trump's family who also met with the Pope. Photo / AP

Still, experts believe it unlikely the outspoken pope will do anything but be welcoming during his first meeting with Trump.

The pontiff said last week he would "never make a judgment about a person without hearing him out" and some Vatican observers suspect he will hold his tongue, at least for now.

"I think that the climate (of the meeting) will be quite good. Because I think there is a mutual interest to close all the polemics of the past and to start working together," said Massimo Franco, author and political analyst for leading daily Corriere della Sera.

He also said the thought a successful staging of the visit could provide Trump with a good news storyline to briefly overshadow the tumult back home over the firing of his FBI director and the ongoing Russia probe.

Trump's visit to the Eternal City comes after two stops in the Middle East where he visited the cradles of Islam and Judaism.

Melania Trump and Donald Trump were welcomed to the Vatican by Pope Francis. Photo / AP
Melania Trump and Donald Trump were welcomed to the Vatican by Pope Francis. Photo / AP

In Saudi Arabia, he addressed dozens of Arab leaders and urged them to fight extremists at home and isolate Iran, which he depicted as menace to the region.

And in Israel, Trump reaffirmed his commitment to strong ties with the nation's longtime ally and urged both the Israelis and the Palestinians to begin the process of reaching a peace deal. No details or timetable have yet to be established for negotiations.

But while Trump received extravagantly warm welcomes in Riyadh and Jerusalem, the reception could grow much cooler now that he's reached Europe, site of widespread protests after his election.

Climate change activists projected the words "Planet Earth First" on the massive dome of St. Peter's Basilica at the Vatican Tuesday night and protests are expected in Rome later in the week when Trump travels to Brussels for a Nato meeting and Sicily for a G7 gathering.

- Associated Press writer Nicole Winfield contributed reporting.