The two had a private 30-minute meeting laden with religious symbolism and ancient protocol.

United States President Donald Trump has met 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.

Mr. Trump, midway through his nine-day maiden international journey, called upon the pontiff at the Vatican early Wednesday 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 8 a.m. 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.

The men shook hands and Mr. Trump could be heard thanking the Pope and saying it was “a great honour” to be there. They then posed for photographs and then sat down at the papal desk, the pope unsmiling, as their private meeting began.

It ended a half hour later when Pope Francis rang the bell in his private study. The pontiff was then introduced to members of Mr. Trump’s delegation, including his wife Melania, daughter Ivanka and son-in-law Jared Kushner, as well as aides Hope Hicks and Dan Scavino. As is tradition, the Pope and the President then exchanged gifts.

The meeting could provide powerful imagery to Catholic voters back in the U.S. as well as the possibility for conflict between a President and a Pope who have not often seen eye-to-eye.

The two men’s often opposite world views collided head-on early last year, when Pope Francis was sharply critical of Mr. Trump’s campaign pledge to build an impenetrable wall on the Mexican border and his declaration that the US. should turn away Muslim immigrants and refugees.

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

Mr. 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 Pope Francis “disgraceful” for doubting his faith.

And even the pontiff’s congratulatory message sent to mark Mr. Trump’s inauguration contained a sly reference to their disagreement, as the Pope wrote that he hoped the U.S. international stature would “continue to be measured above all by its concern for the poor, the outcast and those in need.”

U.S. President Donald Trump, first lady Melania Trump and daughter Ivanka Trump, left, meet with Pope Francis, Wednesday, May 24, 2017, at the Vatican. As is tradition, the pope and president then exchanged gifts. Mr. Trump’s visit to the Eternal City comes after two stops in the Middle East where he visited the cradles of Islam and Judaism. Trump, midway through his grueling nine-day maiden international journey, called upon the pontiff at the Vatican early Wednesday where the two had a private 30-minute meeting laden with religious symbolism and ancient protocol. The president greeted Francis in Sala del Tronetto, the room of the little throne, on the second floor of Apostolic Palace Wednesday morning. Mr. Trump is the 13th president to visit the Vatican and, as part of his tour, he will view the Sistine Chapel. Donald Trump and his wife Melania are welcomed by Prefect of the Papal Household, Mons. Georg Gaenswein, as he arrives at the San Damaso courtyard for his private audience with Pope Francis, at the Vatican, Wednesday, May 24, 2017. U.S. President Donald Trump visits the Western Wall, Monday, May 22, 2017, in Jerusalem. He toured the Church of the Holy Sepulchre, which by Christian tradition is where Jesus was crucified and the location of his tomb. Wearing a black skullcap, he became the first sitting president to visit the Western Wall in Jerusalem’s Old City, the most holy site at which Jews can pray. Melania Trump touches the Western Wall, Judaism's holiest prayer site, in Jerusalem's Old City Monday, May 22, 2017. U.S. President Donald Trump (L) embraces Israel's Prime Minister Benjamin Netanyahu after his remarks at the Israel Museum in Jerusalem May 23, 2017. 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. U.S. President Donald Trump and first lady Melania lay a wreath during a ceremony commemorating the six million Jews killed by the Nazis in the Holocaust, in the Hall of Remembrance at Yad Vashem Holocaust memorial in Jerusalem May 23, 2017. They were joined by Israeli Prime Minister Benjamin Netanyahu and his wife, Sara, as well as Trump’s daughter Ivanka and son-in-law Jared Kushner, who are both White House senior advisers. A still image taken from video shows Yad Vashem Chairman Avner Shalev presenting to U.S. President Donald Trump and first lady Melania a token of remembrance, an exact replica of the original Holocaust-era personal album that belonged to Ester Goldstein, who perished in the Holocaust, at Yad Vashem Holocaust memorial in Jerusalem May 23, 2017. U.S. President Donald Trump (2nd L), senior advisor Jared Kushner (3rd L), first lady Melania Trump (C) and Ivanka Trump, the president's daughter, listens to Rabbi Shmuel Rabinovitch (R) during a visit to the Western Wall, the holiest site where Jews can pray, in Jerusalem’s Old City on May 22, 2017. U.S. President Donald Trump and first lady Melania Trump walk in a procession to the Church of the Holy Sepulchre in Jerusalem May 22, 2017. White House senior advisor Jared Kushner (C) talks to his wife Ivanka Trump during a welcoming ceremony for her father U.S. President Donald Trump at Ben Gurion International Airport in Tel Aviv, Israel May 22, 2017. First lady Melania Trump visits GE All women business process service center in Riyadh, Saudi Arabia, May 21, 2017. In Saudi Arabia, Donald Trump addressed dozens of Arab leaders and urged them to fight extremists at home and isolate Iran, which he depicted as menace to the region. U.S. first lady Melania Trump speaks with Saudi Arabia's Crown Prince Muhammad bin Nayef during an arrival ceremony for U.S. President Donald Trump at the Royal Court in Riyadh, Saudi Arabia May 20, 2017. President Donald Trump holds a sword and dances with traditional dancers during a welcome ceremony at Murabba Palace, Saturday, May 20, 2017, in Riyadh.

Mr. Trump arrived in Rome on 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. He spent the night at the U.S. Ambassador’s residence.

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

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

In recent days, Pope Francis and Mr. Trump have been in agreement on 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 need for dialogue with Muslims. Pope Francis also differs sharply with Mr. Trump on need to combat climate change and economic inequality. And he could react to events this week, including the release of Mr. Trump’s budget, which would dramatically cut funding to programs that help the poor, and the President’s agreement to sell military equipment to Saudi Arabia,.

Still, experts believe it unlikely the outspoken Pope will do anything but be welcoming during his first meeting with Mr. 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. The thought a successful staging of the visit could provide Mr. 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, he said.

Mr. Trump’s visit to the Eternal City comes after two stops in the Middle East where he visited the cradles of Islam and Judaism. 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, he 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 Mr. 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 on Wednesday in Rome and later in the week when Mr. Trump travels to Brussels for a NATO meeting and Sicily for a G7 gathering.