WASHINGTON >> As one of the lead Secret Service agents for President Barack Obama on Inauguration Day in 2009, Joseph P. Clancy experienced the difficulty of protecting someone surrounded by throngs of supporters who want to get close to a leader they adore.

But those crowds seemed tame compared with what Clancy, now the agency’s director, says he saw this year trailing Pope Francis and his security detail in Vatican City as they waded into an ecstatic crowd at St. Peter’s Square.

Clancy and other senior Secret Service officials had traveled there to learn how the pope interacts with audiences and to meet with Vatican security officials about provisions for guaranteeing the pope’s safety during his trip to the United States — a pilgrimage that poses one of the biggest security challenges of his 2-1/2-year papacy.

"The crowds throw things," Clancy, who took over as the head of the Secret Service last October, said in an interview. "They throw flags, dolls and, obviously, babies towards him. It’s difficult because we don’t want our agents to overreact. He doesn’t want to see an overreaction, either, but you don’t want to miss anything."

When Francis arrived in the United States on Tuesday for a six-day visit, the Secret Service took the lead in what is considered by national security experts to be excruciatingly difficult: protecting a pontiff who does not want to stay in his popemobile.

The Secret Service will not discuss how many agents and police officers will take part in protecting Francis during the trip, which will include stops in Washington, New York and Philadelphia. But with crowds expected to number in the hundreds of thousands, federal officials say it will be among the largest mobilizations of security officers in American history.

During Clancy’s visit and in other discussions between the Secret Service and the Vatican, the pope’s aides insisted that Francis wanted to be able to stop his motorcade at any point to pop out and mingle with people on the street. Just as he has tried to make the Roman Catholic Church more open and welcoming, Francis has made it a hallmark of his tenure to shed formalities and reach out to surprised strangers, on the phone and in person.

Francis, who has become known as the People’s Pope, moves about much more freely than his predecessors did. When John Paul II, who survived an assassination attempt early in his papacy, and Pope Benedict XVI appeared before crowds, they often remained in one of the many specially outfitted vehicles known as popemobiles.

To try to protect Francis, officials will be taking several unusual measures.

Large holding pens will be erected along the pope’s motorcade routes for onlookers who have been screened for weapons and explosives. No selfie sticks will be allowed near the pope.

In New York, it will be illegal to operate a drone, and there will be no postal service in some areas. In Washington, the authorities expect so much gridlock that many federal employees have been instructed to take a September "snow day" and work from home.

Francis brought his own security detail, led by Domenico Giani, the inspector general of the Vatican police and security. His security team also includes members of the Swiss Guard, the 500-year-old army of men who take an oath to protect the pontiff. The Secret Service, aware that the Argentine pope is not a fluent English speaker, will always have an agent nearby who speaks Spanish.

To prepare for the pope’s interaction with crowds, the Secret Service held special training sessions for its agents at its campus in rural Maryland.

"It gives our agents a chance to jell and learn how to communicate with each other and read each other’s body language," Clancy said, referring to the security detail teams that have been created for the visit. "The repetition makes them sharper."

From the start of his papacy, Francis has cast himself as an ordinary man, wading into crowds during public events at St. Peter’s Square. He has even accepted glasses of Argentine tea from strangers. All of this has bolstered his global celebrity, drawing waves of fans.

On his visits to Ecuador, Bolivia and Paraguay in July, Francis moved extensively through city streets in open popemobiles, in some cases with limited protective glass. Masses of people gathered behind temporary fencing, and the police presence was heavy. But Francis was exposed, and many people threw flowers and other objects of adoration at his open vehicle as his Vatican security guards jogged beside the moving car.

Vatican officials say that the pope has insisted on using an open-air vehicle in some instances in the United States. After Francis meets with Obama on Wednesday, he plans to ride in his popemobile in a parade in the area that surrounds the White House. In New York on Friday, he will take the vehicle through Central Park.

During his 2013 trip to Brazil, Francis was greeted at an airport in Rio de Janeiro by President Dilma Rousseff. After a welcoming ceremony, Francis clambered into the back of a Fiat hatchback, with the window rolled down. After his driver diverted from his route to avoid traffic, the Fiat was quickly mobbed by well-wishers.

"When I’m going down the street, I wind the window down, so I can put out my hand and greet people," he explained later to Brazilian television. "It’s all or nothing. Either you make the journey as you have to make it, with human communication, or you shouldn’t make it at all."

But Francis also apologized to Brazilian and Vatican security teams who complained that his loose style complicated their work.

"They both know it’s not because I wish to be a naughty child," he said, "but because I am coming to visit people and I want to treat them as people — touch them."

Asked if he felt vulnerable, he replied: "I’m not aware of being afraid."

In a March interview with a community newspaper, Francis was more reflective of his mortality, and his fear of physical pain.

"Look, life is in God’s hands. I told the Lord: ‘You are taking care of me,’ " he told the newspaper. "But if your will is that I die or that they do something to me, I ask you only one favor: that it doesn’t hurt," he said in a light tone. "Because I’m a big coward when it comes to physical pain."

In New York, the authorities are leaving some streets vacant in case there is an episode requiring them to quickly move the pope.

"This is a Broadway play," said Lt. Eugene Whyte of the New York Police Department, who has played a key role in coordinating the visit. "Everything is scripted, and it takes place in Manhattan. It’s not Broadway, it’s all of Manhattan, and it’s all choreographed. But you’ve got to be able to call an audible."

"You have to," he said. "You have to be able to go."