It's official.

Ending months of "will-he-or-won't-he" speculation, Pope Francis confirmed early Monday that he will come to Philadelphia next fall for the World Meeting of Families.

And the news sent a jolt of excitement through the city on a morning that looked like it was going to be overshadowed by the Eagles' devastating loss to the Green Bay Packers.

Before a crowd of more than a hundred roaring Catholic school students, Mayor Nutter said the visit will be the largest event in the city's modern history.

"We are all tremendously excited," he said. "We are fully engaged, fully involved and fully invested in this incredible effort."

Organizers said Monday the pope is expected to be here for the last two days of the five-day event and that up to two million people could pack Center City for a papal Mass on the Parkway on the last day, Sept. 27.

It will be Francis' first scheduled visit to the U.S. since the Argentine Jesuit became pope in March 2013.

It was widely assumed Francis would come to Philadelphia. But the official announcement from the Vatican came shortly after 3 a.m. Philadelphia time and nevertheless generated buzz in the city. The pope also has been invited to address the United Nations General Assembly, also held in September, and to visit Washington.

Nutter indicated trips to New York and Washington also were a possibility, creating what he called a "massive undertaking."

Catholics attending Mass this morning at the Basilica of Saints Peter and Paul rejoiced with the news of Pope Francis' trip to the city.

While Francis' attendance at the World Meeting of Families had long been rumored, audible gasps were heard in the church when the visit was announced at the beginning of mass. One woman couldn't help herself from quietly clapping in her pew.

"It's fantastic," Diane Dabis said after the service, adding she hoped the pope would "shed his usual light on the city."

"I think it's awesome," Tatiana Lima said. She's already planning to attend the pope's expected Mass. The massive crowds and long lines won't be a problem, Lima said.

"I'm an early riser," she said.

Charles J. Chaput, Archbishop of the Philadelphia Archdiocese, was present for the the announcement at the Vatican.

"I applauded the loudest," he told the Associated Press.

"I am overjoyed by Pope Francis' announcement that he will join with us for the World Meeting of Families in Philadelphia next year," Chaput said in a prepared statement. "A hallmark of his papacy has been a keen focus on the many challenges that families face today globally. His charisma, presence and voice will electrify the gathering."

"As I've said many times before, I believe that the presence of the Holy Father will bring all of us – Catholic and non-Catholic alike – together in tremendously powerful, unifying and healing ways," Chaput continued. "We look forward to Pope Francis' arrival in Philadelphia next September and we will welcome him joyfully with open arms and prayerful hearts."

Mayor Nutter, an honorary co-chair of the World Meeting of Families, and other officials involved in organizing the World Meeting voiced their excitement at a news conference at the Art Museum with a rain shroud as visible backdrop.

"This will be the largest event of the city's modern history," said Nutter, who acknowledged knowing the pope's plans Sunday but could not even tell his mother, a Catholic high school graduate, about it.

Pennsylvania First Lady Susan Corbett said that during a trip earlier this year to invite the pope to Philadelphia, Francis whispered something in Gov. Corbett's ear.

"I will come," she said he told him, adding they, too, had been sworn to secrecy.

"We look forward to sharing this amazing experience with you," Susan Corbett said.

Donna Crilley Farrell, executive director for the World Meeting of Families 2015, said: "What a great and historic day for Philadelphia, for Pennsylvania, and for our entire country"

Robert Ciaruffoli, president of the organizing group, said, "it's a moment we've all been waiting for."

"We must, and we will, provide a flawless series of events," he said. "The planning has already started." He put the budget at $45 million.

The conference is billed as the largest global gathering of Catholic families, held every three years. The 2015 conference will be the first one held in the United States, following seven previous events in Italy, Spain, Mexico, Brazil and the Philippines.

"The World Meeting of Families is an opportunity for people of all faiths to have important conversations and engage spiritually," Nutter said "Strengthening families is a mission that all of us can, should and must support."

The pope's presence is sure to boost an already massive event. The conference is expected to generate tens of millions of dollars in spending at local restaurants, hotels, retail shops, and tourist attractions — and cost tens of millions for security, logistics, space and other needs.

A nonprofit foundation led by Pennsylvania religious and civic leaders has been heavily engaged in planning and fund-raising since February 2013, when Philadelphia was selected — and before Cardinal Jorge Mario Bergoglio was elected to succeed Pope Benedict.

In March, a delegation including foundation co-chairs Mayor Nutter and Gov. Corbett went to Rome to personally invite the newly inaugurated Pope Francis to Philadelphia. Just last week, students from archdiocesan schools sent some 10,000 letters to the Vatican, urging the pontiff to come, the Daily News reported.

In recent months there have been numerous indications that the pontiff would indeed make the trip. The pope himself said in August that he wanted to attend the conference, and perhaps also travel to New York to speak at the United Nations and to Washington to address Congress.

Chaput and the Vatican's U.N. envoy also hinted that Pope Francis would come. But the Vatican hedged all of those indications with a warning that the pope had "no concrete plans" yet.

Monday's announcement, at a "Humana Colloquia" at the Vatican, was the "concrete plan" organizers have been eagerly awaiting.

Gavin said the archdiocese was scrambling this weekend to prepare for the announcement. Chaput was invited to the Vatican to participate in the colloquia, but he was not told until Saturday afternoon that the confirmation would be made there, Gavin said. Chaput will remain there through the end of the week.

The last time a pope visited Philadelphia was in 1979, when John Paul II drew more than a million people to the Benjamin Franklin Parkway to attend Mass.

In his short tenure, Pope Francis has become a popular figure, even among non-Catholics. But he has rankled some conservative Catholics by opening up for debate long-standing church doctrine about homosexuality, birth control and divorce.

Although the Vatican ultimately sets the agenda for the World Meeting of Families, Chaput said it was unlikely that the conference would tackle such topics as same-sex marriage and the church's ban on Catholics who remarry after divorce receiving Holy Communion.

"All of us have some divorced and remarried people in our families or in same-sex relationships," Chaput said in March, while visiting the Vatican. "But we're not going to focus on problems and conflicts" or provoke uncertainty.

Chaput said the conference "will deal with a wide range of family issues where our faith is both needed and tested," including marriage, poverty, addiction, disabilities, and the effects of divorce.

Registration for the event opened earlier this month.

---

Jessica Parks and Tricia Nadolny are Inquirer staff writers.

Emily Babay is on the Philly.com Breaking News Desk

215-854-2443

breakingnewsdesk@phillynews.com