White worked for the Canadian Security Intelligence Service and was involved in coordinating security measures for the Olympic Games and other large-scale events.

Mayor Nutter on Wednesday announced a plan to ban vehicle traffic within Center City, close the Vine Street Expressway and other major highways, and close the Ben Franklin Bridge to vehicle traffic. SEPTA earlier announced its plans to close all but 21 train stations and require special papal passes to travel on Regional Rail trains.

Henry Willis, director of the RAND Homeland Security & Defense Center in Pittsburgh, said: "You have to do security in a way that doesn't ruin the primary purpose of the event. You want to try to not disrupt the city too much.

"You've got to balance control, based on what you know, versus the purpose of the visit and the disruption that you're causing."

He said that it was common practice by cities to briefly close highways or bridges during the movement of a VIP such as the president or pope, but much less common to close them for extended periods of time.

Sophisticated terrorists could elude many of the security measures during the papal visit, and "lone wolf" actors might be undeterred by them, some experts said.

Terrorists could simply come to Center City months or weeks in advance of the pope's visit on Sept. 26 and 27 and wait, safely within the security ring.

A lone-wolf attacker might be stopped from attacking the pope, experts say, but not from exploding a bomb or firing into a crowd waiting at a security checkpoint.

"It's virtually impossible to set up a police perimeter around a crowd that large," said Edward Davis, who was Boston police commissioner at the time of the Boston Marathon bombing in 2013. "You have to concentrate your security on certain points."

Typically, security is based on concentric rings of protection around the protected person, with increasingly intense security near the VIP.

"People closer to the pope will have been screened . . . while those farther away probably will be separated by fences," said Davis, who is now president of a security consulting company, Edward Davis L.L.C. "They're not going to be able to screen everyone."

Everett Gillison, deputy Philadelphia mayor for public safety, said Friday that the papal visit comes with "huge logistical and security responsibilities."

"The U.S. Secret Service and the city, working with many other partners, are developing very detailed plans, some of which have been completed and made public," Gillison said in a statement.

"At its core, the multifaceted plan balances the desire for a smooth running series of events involving perhaps 1.5 million faith visitors to the City with vital security and logistical requirements that 21st Century events demand.

"At a very practical level, the huge crowds expected will be walking on narrow city streets and sidewalks. To accommodate this unprecedented influx of people, thoroughfare closures are mandatory.

"Allowing vehicles to attempt to move through large crowds would be an invitation to disaster. Further, providing emergency services to more than a million visitors requires dedicated emergency routes free of civilian vehicle traffic."

Gillison dismissed the "instant 'analysis' from those who know little about the complexity of the approaching events."

Secret Service spokesman Robert Hoback said others"haven't seen our plan, since we haven't released it." He said the city was responsible for the plan announced Wednesday by the mayor that described the no-drive zone and highway and bridge closures.

Hoback said the Secret Service security plan for the "innermost secure perimeter, where the pope will be," will be released about three weeks before the pope's arrival.

"The Secret Service, in conjunction with its law enforcement and public safety partners, will publish a list of prohibited items once the information is finalized," Hoback said in a statement Friday. "This list of prohibited items will only apply to visitors who are accessing specific papal venues in Center City."

Jeff Tomlinson, a former supervisory special agent for the FBI in Philadelphia who was the supervisor of the Philadelphia Joint Terrorism Task Force, said he was not privy to specific threats that law enforcement agencies may have, so he wouldn't second-guess their actions.

"I can't speak as to whether it's an overreaction by the Secret Service," he said. The security perimeter "looks like a big area, but that might be the right way to do it."

"It's better to have over-prepared and nothing happens than the flip side," Tomlinson said. "The mind-set now is to be proactive rather than reactive."

Despite the high profile of foreign terrorist groups such as al-Qaeda and ISIS, Tomlinson said it was vital not to forget about "domestic threats and lone individuals."

Heavy security might discourage lone wolves, Tomlinson said.

White said that individual attackers might take advantage of crowds at choke points, such as at the Camden side of the Benjamin Franklin Bridge, which is to be closed to all but foot traffic during the papal visit.

The massive steel bridge would not likely be harmed by an individual with an improvised explosive device, but people queued up for screening could be, White said.

"We can keep pushing our perimeters farther and farther out, but if I'm carrying a backpack with an explosive, maybe I'll just walk up to the choke point and detonate there.

"Does it make a difference to the families if a person is killed on the New Jersey side of the bridge?" White said.

Law enforcement officials familiar with security plans have said that all bridge travelers will be screened on the Camden side of the bridge.

The Secret Service said Friday that neither the Secret Service nor the City of Philadelphia would be screening papal visitors on the bridge. It did not address any screening by other law enforcement agencies, such as Camden County police.

The Delaware River Port Authority, which operates the bridge, has declined to disclose its security plans for the bridge.

"What visitors should expect is some type of law enforcement presence on the bridge to ensure those who wish to cross, can do so in a safe and expeditious manner," according to the Secret Service statement.

"We don't have anything to do with security at the Ben Franklin Bridge. Why would we?" Hoback said in Friday's statement. "We don't control the bridge."

John Hanson, the chief executive of the DRPA, publicly complained last week that the DRPA and other agencies were frustrated about their inability to make security decisions, until the Secret Service announced what it was going to do.

Davis, the former Boston police commissioner, said security for large-crowd events with important dignitaries "is very much a balancing act" between protection and freedom of movement.

"The scale tilts toward protection when you have someone like the pope," Davis said. "I can't think of a more important person. . . . This is probably the height of extreme measures. You're probably going to see unprecedented security."

Sometimes, highways and bridges may be closed to traffic, experts say, to allow quick movement of emergency vehicles, or to provide for safe exits for crowds if something goes wrong at a large public event.

Meanwhile, security has intensified since the last papal visit to Philadelphia in 1979, because of terrorism fears brought on by the Sept. 11 strikes and other high-profile attacks.

And technology has advanced, giving new tools both to terrorists and security agencies.

If the goal of the security efforts for the pope's Philadelphia visit is to protect two million visitors, White said, the heavy restrictions "will protect them from a large truck bomb that could kill thousands, but the individual actor with a firearm or explosive device is almost impossible to prevent."

The bottom line, White said, is: "What risk are we prepared to live with?"