Pope Francis issued a ringing call to action on behalf of immigrants, urging politicians at the US Congress to embrace "the stranger in our midst" as he became the first pontiff in history to address a joint meeting of the legislators.
Referencing the migration crisis in Europe as well as the United States's own struggle with immigration from Latin America, Francis summoned politicians "to respond in a way which is always humane, just and fraternal".
"We must not be taken aback by their numbers, but rather view them as persons, seeing their faces and listening to their stories, trying to respond as best as we can to their situation," Francis urged.
He was welcomed enthusiastically to a House chamber packed with Supreme Court justices, cabinet officials and politicians of both parties, uniting the bickering factions before he even opened his mouth as all stood to cheer his arrival.
The sergeant at arms intoned "Mr Speaker, the pope of the Holy See," and Francis made his way up the centre aisle in his white robes, moving slowly as politicians applauded, some lowering their heads in bows.
After the speech, he appeared on a Capitol balcony and briefly addressed a cheering crowd of thousands below on the lawn and the Mall beyond.
"Buenos dias," he called out, and the crowd thundered its response. "God bless America!" he concluded, as he had in the House chamber.
Thursday's speech was the latest highlight for the pope's whirlwind three-day visit to Washington, the first stop on a three-city US tour.
On Wednesday he was cheered by jubilant crowds as he visited the White House - where he and US president Barack Obama embraced each other's warnings on climate change - paraded through Washington streets in his "popemobile," addressed US bishops, noting the clergy sex abuse scandal, and celebrated a Mass of Canonisation for Junipero Serra, the Spanish friar who founded major California missions.
Later on Thursday he moves on to New York and then later in the week to Philadelphia.
Introducing himself at the Capitol as "a son of this great continent", the Argentine pope, reading his remarks slowly in English, spoke from the same dais where presidents deliver their State of the Union speeches.
Behind him sat vice president Joe Biden and House Speaker John Boehner, the first and second in line to the presidency, both Catholics.
Outside, tens of thousands watched on giant screens and many more were watching on TV around the world.
Politicians of all political backgrounds and religious affiliations eagerly welcomed the pope, pledging to pause from the bickering and dysfunction that normally divide them to listen to the pope.
Yet Francis spoke to a Congress that has deadlocked on immigration legislation, at a time when there are more than 11 million people in the US illegally, and where some politicians have balked at Obama administration plans to accept more of the migrants from Syria and elsewhere who are now flooding Europe.
Indeed, Francis arrived at a moment of particular turmoil for Congress, with a partial government shutdown looming next week unless politicians can resolve a dispute over funding for Planned Parenthood related to the group's practices providing foetal tissue for research.
Mr Boehner himself, who invited Francis to speak and met with him privately beforehand, is facing a brewing revolt from tea party members who have threatened to force a floor vote on whether the Speaker can keep his job.
Francis steered clear of such controversies, alluding only in passing to the Catholic Church's opposition to abortion when he noted, to applause, "our responsibility to protect and defend human life at every stage of its development".
He advocated abolition of the death penalty, something that enjoys support from a number of politicians of both parties at the federal level, and spoke out against fundamentalism of all kinds, while urging care in combating it.
"A delicate balance is required to combat violence perpetrated in the name of a religion, an ideology or an economic system, while also safeguarding religious freedom, intellectual freedom and individual freedoms," Francis said.
On immigration, Francis urged politicians - and the United States as a whole - not to be afraid of migrants but to welcome them as fellow human beings, not things that can be discarded just because they are troublesome.
Francis, the son of Italian immigrants to Argentina, recalled that America itself was founded by immigrants, that many politicians are descended from foreigners and that new generations must not "turn their back on our neighbours".
Given an ovation when he spoke of the Golden Rule, he said: "Let us treat others with the same passion and compassion with which we want to be treated."
Ahead of Francis's remarks, politicians of both parties had busily sought political advantage from his stances, with Democrats in particular delighting in his support for action to combat global warming. One House Republican back-bencher announced plans to boycott the speech over Francis's activist position on climate change.
Francis reiterated that stance on Thursday, urging action to address "the environmental deterioration caused by human activity".
"I am convinced that we can make a difference and I have no doubt that the United States - and this Congress - have an important role to play," he said.
Many politicians had vowed to preserve decorum throughout the speech and members of both parties listened intently, yet they did not completely contain their reactions. The mention of climate change drew standing cheers from Democrats while Republicans stood to applaud the reference to abortion. One Democratic House member let out a whoop of delight at the pope's call to abolish the death penalty.
Republicans in particular also loudly applauded as Francis asserted the importance of family life and bemoaned that "fundamental relationships are being called into question, as is the very basis of marriage and the family". The Catholic Church opposes gay marriage, recently legalised by the Supreme Court.
Francis also criticised the arms trade, significant before Congress because the United States is the world's largest exporter of weapons.
Security was tight outside the Capitol, with streets blocked off and a heavy police presence that rivalled an inauguration or State of the Union address by the US president. The scene on the West Lawn was festive but orderly.
Francis enjoys approval ratings the envy of any US politician as he has remade the image of the Catholic Church towards openness and compassion, yet without changing fundamental church doctrine. Addressing a chamber full of elected officials, he may have been the most adept politician in the room.
After speaking in the House chamber, Francis stopped at the Capitol's Statuary Hall and its statue of Father Serra.
Later, he planned to stop at St Patrick's Catholic Church and the Catholic Charities of the Archdiocese of Washington, before leaving for New York for more prayer services and a speech to the United Nations.