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Democratic presidential hopeful Bernie Sanders met with Pope Francis in the Vatican on Saturday, calling the short visit an "honor and a joy."

"Today certainly was the highlight of the trip," Sanders told NBC News of his 6 a.m. (midnight ET) meeting with the pontiff outside the papal residence's breakfast room.

He called the pope "one of the great leaders in this world."

The meeting between the Pope & @BernieSanders happened around 6AM this morning & @Pontifex spoke in both Eng & Span. pic.twitter.com/HmOieUGUO8 — Danny Freeman (@DannyEFreeman) 16 April 2016

"In terms of economic justice, the need to combat climate change — nobody in the world has been more profound and articulate than the pope, and I'm delighted to go forward with him on those issues," said Sanders, who has made wealth inequality a cornerstone of his campaign.

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Sanders was at the Vatican to attend a meeting about social justice and economic issues. Later on Saturday, Pope Francis flew to the Greek island of Lesbos to highlight the plight of migrants and refugees struggling to get into Europe. The two ran into each other in the lobby of the pope's residence, the Domus Santa Marta hotel in the Vatican gardens, where Sanders and his wife, Jane, stayed overnight on the same floor as Francis.

The pope later noted to reporters that members of the Vatican conference that Sanders had attended also were staying at the hotel.

"This morning when I left, Sen. Sanders was there. ... He knew I was leaving at that time and I had the kindness to greet him and his wife and another couple who were with them," the pope told reporters traveling back with him to the Vatican.

.@BernieSanders tells @annenbcnews that he "expressed his great admiration" of the Pope during their meeting today. pic.twitter.com/cLYqXoJ50W — Danny Freeman (@DannyEFreeman) 16 April 2016

"When I came down, I greeted them, shook their hands and nothing more. This is good manners. It's called good manners and not getting mixed up in politics,” the pope said. “If anyone thinks that greeting someone means getting involved in politics, they should see a psychiatrist."

The Vatican is loath to get involved in electoral campaigns, and usually tries to avoid any perception of partisanship as far as the pope is concerned, but Francis has been known to flout Vatican protocol, and the meeting with Sanders was evidence that his personal desires often sidestep Vatican diplomacy.

In February, Francis rebuked Republican presidential candidate Donald Trump over the GOP front-runner’s stand on immigration.

Sanders, who is aiming to continue cutting into rival Hillary Clinton's significant lead in delegates ahead of the crucial New York primary this week, denied that it had been mistake to take time out of his campaign to fly to Europe.

"I am a great admirer of Pope Francis and when I received this invitation, I believe that I would've kicked myself for the rest of my life if I did not attend a conference here in the Vatican to deal with issues that have been important to me for decades," he said.

Sanders has acknowledged he disagrees with the Catholic Church's teachings on some social issues, such as gay marriage, but has praised Pope Francis for "injecting a moral consequence into the economy."

When NBC News asked whether the trip to the Vatican was an attempt to win a "papal endorsement," Sanders responded: "Oh God no. Not, God no!"

Then he laughed, adding: "That just came out."