Pope Francis unveils plan to visit the U.S. next September. (Catholic Church England and Wales/Creative Commons)

The trip marks the pontiff's first to the United States since his election last year.

Francis is due to attend the “World Meeting of Families” in Philadelphia, an event held every three years in different cities, where prelates are expected to discuss family issues. Francis will also hold a Mass, likely on the Benjamin Franklin Parkway, which could draw up to a million people.

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Philadelphia Mayor Michael Nutter said on Monday the event would be the largest in the city's modern history.

"What an historic day, and an unparalleled day for the city of Philadelphia," Nutter said. "We could not be more excited."

The website of the Philadelphia event said the Argentinian pope would visit the city from Sept. 25-27.

He is also expected to address the UN General Assembly in New York and meet President Barack Obama in Washington.

He will be the second pope to visit Philadelphia after John Paul II’s visit in 1979. The last papal visit to the United States was made by Pope Benedict XVI in 2008.

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Francis' announcement was eagerly awaited after months of speculation.

Describing Francis’ visit, Philadelphia Archbishop Charles Chaput said, “His charisma, presence and voice will electrify the gathering.”

Chaput, who had personally invited the Pope, said he was "overjoyed" by the announcement, adding: "A hallmark of his papacy has been a keen focus on the many challenges that families face today globally."

Francis made his announcement of the trip at a Vatican conference on family values at which he delivered a robust defense of the heterosexual family unit.

“The family is the foundation of co-existence and a remedy against social fragmentation,” he said. “Children have a right to grow up in a family with a father and a mother capable of creating a suitable environment for the child's development and emotional maturity.”

The family, added Francis, “is an anthropological fact,” adding, “We cannot qualify it based on ideological notions or concepts important only at one time in history.”

Francis’ speech came two days after he condemned euthanasia and in vitro fertilization, which he called “the scientific production of a child.”

The pope's voyage will come at a time when the U.S. church is trying to keep Catholics in the fold, including Latino immigrants who have been joining Protestant churches or leaving organized religion in significant numbers. Expectations will also be high for Francis to address the clergy sex abuse scandals.

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During a synod in October, bishops split over the Catholic Church's receptivity to gay and unmarried couples.

His recent speeches will go some way to placating U.S. bishops startled by Francis’ seeming pullback from the church's hard-line approach to hot-button issues such as gay marriage, particularly after he said last year, “We cannot insist only on issues related to abortion, gay marriage and the use of contraceptive methods.”

On Monday, a senior U.S. cardinal said Francis needed to choose his words carefully, starting with his now-famous statement in answer to a question about homosexuality: “Who am I to judge?”

That phrase “has been very misused … because he was talking about someone who has already asked for mercy and been given absolution, whom he knows well,” said Cardinal Francis George, who steps down as archbishop of Chicago on Tuesday.

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“Does he not realize the repercussions? Perhaps he doesn’t,” George told the website Crux. “The question is why he doesn’t clarify” the statements?

Next year’s “World Meeting of Families,” at which the pope will likely discuss family issues, will come just weeks before a key synod that will pick up on the divisive themes debated in Rome in October.

This piece is a curation of reporting from the BBC, Los Angeles Times, Associated Press, and Yahoo News.

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