US President Donald Trump is due to hold talks with Pope Francis and Italy’s top leaders in Rome on the third leg of his first oversees trip.
Mr Trump and the pontiff have already clashed at a distance on issues including migration and climate change.
Mr Trump will also meet Italy’s president and prime minister, before flying to Brussels for a Nato summit.
He earlier vowed to help Israelis and Palestinians achieve peace, as he ended the Middle East leg of his tour.
The US leader began his foreign trip with a two-day stop in Saudi Arabia over the weekend, urging Muslim countries to take the lead in combating radicalisation.
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Mr Trump is expected to meet Pope Francis at 06:30 GMT, before holding talks with Italian President Sergio Mattarella and Prime Minister Paolo Gentiloni.
Seeking common ground – analysis by BBC’s Jon Sopel
It is hard to think of two more contrasting characters than Pope Francis and President Trump.
On one hand, the Jesuit who has made his mission the championing of the poor and dispossessed; on the other the property developer who has championed getting rich, and surrounded himself with billionaires in his cabinet.
And though this will be their first meeting they’ve already sparred. During the election the Pope on a visit to the Mexico-US border said that people who only think of building walls instead of bridges were not Christian.
Donald Trump described those comments as disgraceful, and accused the pontiff of being a pawn of the Mexican government.
But on Wednesday both men will be seeking to find common ground.
After his visit to Saudi Arabia, home to Islam’s holiest sites, and to Israel, this is the final leg of the tour of the world’s three great religions.
President Trump’s commitment to fighting extremism and intolerance will win approval from the Pope, as will his determination to bring peace to the Middle East.
And the president thinks there’s another reason why they will get on. Back in 2013 he tweeted: “The new Pope is a humble man, very much like me.”
This is Mr Trump’s first visit to Europe since he took office in January.
Security has been stepped up across Rome, with the areas around the Vatican City, the Italian presidential palace and the American ambassador’s residence, where Mr Trump is staying, temporarily closed to traffic.
Despite the heavy police presence, about 100 anti-Trump protesters held a rally in one of Rome’s squares on Tuesday evening.
Later on Wednesday, Mr Trump will fly to Brussels, where significant protests are expected.
For the EU and for Nato, this visit is about damage limitation with the fervent hope of establishing some kind of transatlantic chemistry, the BBC’s Europe editor Katya Adler says.
She adds that the tone in Brussels has gone from off-the-record sneering when the erratic and unpredictable Mr Trump first won the November elections, to outright concern now that the implications of his presidency have begun to sink in.
Mr Trump will end his tour on the Italian island of Sicily at the G7 summit on Friday.
On Tuesday, he said he would “do everything” to help Israelis and Palestinians achieve peace.
At talks in Bethlehem with Mahmoud Abbas, he spoke of being gratified that the Palestinian leader had committed to taking necessary steps to “fight terrorism”.
Israel and the Palestinians have not held peace talks for three years and Mr Trump acknowledged it would be “one of the toughest deals of all” to broker.
In both Gaza and the occupied West Bank, Palestinians have held protests against the trip and in support of a hunger strike by Palestinian prisoners in Israeli jails.
Mr Trump earlier said he had come to “reaffirm the unbreakable bond” between the US and Israel and that there was a “rare opportunity to bring security and stability and peace” to the region.
He added that the president of the Palestinian Authority had assured him he was “ready to work towards that goal in good faith”, and that Israeli Prime Minister Benjamin Netanyahu had promised the same during their talks in Israel.