Pope Francis blesses people from his popemobile as he travels to the Shrine of the Virgin of Caacupe, in Caacupe, Paraguay, in July 2015. (Jorge Saenz/AP)

Federal authorities are mobilizing one of the largest security operations in U.S. history ahead of Pope Francis’s arrival Tuesday, an effort that is straining law enforcement resources in Washington, New York and Philadelphia.

Hundreds of thousands of onlookers are expected to gather in all three cities for a glimpse of the Catholic Church leader, whose unrivaled global popularity and proclivity to wade into public crowds has added to security concerns. Thousands of federal and local personnel will be deployed to keep the pontiff and the public safe.

The challenges are immense.

The pope’s five-day tour, his first in the United States, will include appearances at the White House and Congress, a parade on Constitution Avenue in Washington, a Mass at Madison Square Garden, a procession through Central Park in New York, and an open air Mass with up to 1.5 million people in Philadelphia.

Pope Francis is seen more favorably than his predecessors among Catholics and non-Catholics according to recent polls. However, when he visits the United States beginning Sept. 22 in Washington, D.C., he will be greeted by a Catholic community that has been losing members. (Jayne W. Orenstein and Julie Percha/TWP)

His Big Apple visit coincides with the 70th U.N. General Assembly, where more than 150 foreign delegations are also expected, creating a virtual lockdown in portions of midtown.

Homeland Security Secretary Jeh Johnson has designated Francis’s stops in each city a National Special Security Event, a rare designation to streamline the federal response that had previously been used for presidential inaugurations, State of the Union addresses, political conventions, NATO summits, the 2002 Winter Olympics and Super Bowl XXXVI in 2002.

The U.S. Secret Service is in charge of coordinating the massive intergovernmental operation on counterterrorism operations, crowd management, crisis response and air and vehicle traffic control. The FBI, Capitol Police, Coast Guard, Pentagon and Federal Emergency Management Agency are closely involved in the planning, along with local police departments.

Federal agencies in Washington are encouraging employees to work from home Tuesday through Thursday to cut down on congestion in the city. But further adding to the security planning challenges, Chinese President Xi Jinping arrives at the White House next Friday for a state visit.

“They’re standing up one of the largest security programs for an individual in U.S. history over three cities,” said Jonathan Wackrow, a former Secret Service agent who assisted in security preparations for Obama’s 2013 inauguration. “The level of protection afforded to Pope Francis is equal to or exceeding that provided to the president of the United States.”

At a news conference in Philadelphia on Friday, Johnson said that federal and local authorities have been planning for months. Officials offered reporters a glimpse of the Multi-Agency Communications Center where up to 90 staff members from 50 agencies will gather around the clock to monitor video, radio and other communications.

“I want to stress the level of preparation and planning and coordination,” said Johnson, flanked by Secret Service Director Joseph Clancy and Philadelphia Mayor Michael Nutter (D). “We have worked hard to achieve what we believe will be a safe and successful event and visit by the Holy Father and a safe and successful time for all those who wish to be here and see him. We believe we have struck that right balance for the public.”

Threats to a pope’s safety are not theoretical. Pope John Paul II was shot and wounded while entering the Vatican square in May 1981.

Behind the scenes, the Secret Service has been mapping out a security strategy for a visit that is virtually unprecedented in nature. Pope Benedict visited the United States in 2008, and John Paul came several times, but Francis has cultivated a peculiar brand of personal popularity with his common touch.

Clancy and the agents who will serve as Francis’s protective detail leaders traveled to the Vatican this summer to meet with the Catholic Church’s own security division. They followed the open-air popemobile around the Vatican Square to observe his movements and met with the pope directly.

“We got a first-hand view of how things worked there,” Clancy said in a brief interview. “It was very beneficial for the detail leaders and for me.”

Secret Service protective teams — including counter-snipers and counter-assault agents — have drilled at training facilities, employing a replica popemobile.

Intelligence agencies have been monitoring foreign and domestic terrorist threat levels. Rep. Michael McCaul (R-Tex.), chair of the House Homeland Security Committee, said publicly last weekend that the Secret Service had disrupted a plot related to the pope’s visit. But other federal authorities said they were not aware of the alleged incident.

Security experts emphasized that protecting the pope is uniquely challenging because of his penchant for mingling directly with spectators in large crowds. In 2013, a passenger vehicle carrying the pope was blockaded by frenzied crowds in Rio de Janeiro. Francis rolled down his window, touching those who reached into the car, and he even kissed a baby passed to him through the window, according to news accounts.

Secret Service agents said that in the past they have been instructed by Vatican officials not to manhandle the pope in rope lines; with U.S. presidents, agents are accustomed to holding onto a belt or suit coat to be able to evacuate him quickly.

Congressional leaders sent a memo to lawmakers this week instructing them not to reach out to shake hands or touch Francis as he walks through the House chamber to deliver remarks next Thursday.

“I was never more moved than by what I saw of the effects the pope has on people,” said Ralph Basham, a former Secret Service director who was assigned as the lead protective agent for John Paul’s visit to Denver in 1993. “It makes it very, very stressful and challenging for us. I’ve never been anywhere where there was more emotion than when I saw that in Denver. The only other time I did see such frightening emotion was in Egypt when [President] Nixon visited Cairo” in 1974.

The extraordinary precautions have prompted security experts to question whether the inconveniences could become extreme and counterproductive in some cases.

Scott White, an associate professor of national security at Drexel University in Philadelphia, called that city’s plan to close highways and bridges for the duration of the pope’s visit potentially unprecedented. Even visits by presidents require only temporary shutdowns, he noted.

“I don’t know what scenario they’ve strung together to come up with that plan of action,” White said.

Kurt L. Schmoke was mayor in Baltimore when John Paul visited in 1995 for a downtown parade and a Mass before 60,000 at Oriole Park at Camden Yards.

Snipers were positioned on rooftops, mail and newspaper boxes were removed, and manhole covers were welded shut — and that was in an era before the 9/11 terrorist attacks. Schmoke said in a recent interview that the elaborate security precautions were worthwhile.

“It was a major coordination between multiple levels of governments and security,” he said. “I don’t recall us shutting down the city.  . . . The biggest issue was who was going to get tickets to the stadium.”

The enormous costs of the security operations have raised additional concerns within federal agencies and local law enforcement departments, whose operating budgets do not necessarily have sufficient contingencies for significant overtime charges. The federal government budgets $4.5 million a year to handle National Special Security Events, Clancy said, but officials did not clarify whether that was enough to cover the pope’s entire visit. It is unlikely to cover some fees incurred by local law enforcement agencies, experts said.

“The papal visit will be significant in the planning and scope, almost similar literally to an inauguration, perhaps even surpassing that,” U.S. Capitol Police Chief Kim Dine testified at a Senate appropriations subcommittee hearing in March. “That is a huge event and something frankly that goes above and beyond our budget.”

In the end, though, some believe the Holy Father’s safety might be in the hands of a higher power. Basham recalled flying on a Marine helicopter with John Paul in Denver when the pontiff began to make the sign of the cross and seemed to disappear into “a trancelike state.”

“All of a sudden, he went somewhere else for a minute,” Basham said. “Quite frankly, a couple times I was convinced he was having a private conversation with someone. I can only assume it was with God, I guess.”