MANILA - Philippine troops are facing a security "nightmare" during Pope Francis's visit starting Thursday, with potential stampedes, Islamic militants and lone-wolf assailants all concerns.
Nearly 40,000 soldiers and police are being deployed to protect the pontiff during his five-day trip to the Philippines, a majority Catholic nation where attempts have been made to kill visiting popes twice before.
"For this year, this will be the greatest security nightmare that we can have," Philippine military chief General Gregorio Catapang said as he readied his troops for the pontiff's arrival.
Authorities have stated the huge crowds of devout Catholics are their main worry, with up to six million people expected for a mass in Manila on Sunday.
Giant throngs are also expected along his motorcade routes in the capital, while a one-day trip to typhoon-devastated communities in the central Philippines will pose its own problems.
In a nationally televised address on Monday, President Benigno Aquino pleaded with his countrymen planning to join the crowds to remain calm and avoid creating a stampede that could endanger the pope.
"I ask you, do you want history to record that a tragedy involving the pope happened in the Philippines," Aquino said.
Aside from a crowd potentially crushing the pope, Aquino also warned that blocking his motorcade would make him an easy target for an assailant.
The president referred to the threat of "terrorism" and an assassination attempt on John Paul II at the Vatican in 1981, as he called on all Filipinos to help protect Francis in the Philippines.
Adding to the concerns, the 78-year-old pontiff has insisted he will not travel in a bullet-proof "popemobile" so can he be closer to his flock.
Highlighting the priority Aquino has placed on security, he personally led a late-night dry run on Tuesday of the motorcade journey that the pope will make from the airport after he arrives - with thousands of police lining the roads.
There have also been two attempts or plots to kill pontiffs visiting in the Philippines that Aquino did not refer to in his televised address.
On the first-ever papal visit to the Philippines in 1970, Bolivian painter Benjamin Mendoza donned a priest's fake cassock and swung a knife at Pope Paul VI as he arrived at Manila airport.
Paul VI was wounded but continued his trip without disclosing his injury.
Then, one week before John Paul II's visit in 1995, police uncovered a plot by foreign Islamist extremists to kill him by bombing his Manila motorcade route.
They then planned to set off explosives on 11 US jetliners over the Pacific Ocean that they hoped would kill thousands.
The plot was detected only because bomb-making material caused smoke at the apartment that was being used to store the explosives.
Pakistani Ramzi Yousef, who carried out the 1993 World Trade Center bombings in the United States, and Khalid Sheikh Mohammed, one of Al-Qaeda's most senior figures who planned the September 11, 2001 attacks, were among those involved in the plot.
Aside from foreign extremists, Philippine security forces have for decades struggled to contain local Islamic militants with links to Al-Qaeda.
The most well-known, the Abu Sayyaf, operates mostly on southern islands populated by the nation's Muslim minority many hundreds of kilometres (miles) from Manila.
But it is accused of carrying out the Philippines' deadliest terrorist attack, the bombing of a ferry in Manila in 2004 that killed more than 100 people.
Senior Abu Sayyaf senior leader Khair Mundos, who was on the US government's most wanted list, was also arrested while living at an apartment near the Manila airport just seven months ago.
While the government has insisted no specific plots have been detected against the pope, he is undoubtedly a tempting target for Filipino Islamic militants, according to Rommel Banlaoi, director of the Philippine Institute for Peace, Violence and Terrorism Research, a Manila think-tank.
"For violent groups opposed to the rule of the Catholic Church, attacking the pope is like a trophy, a major accomplishment," Banlaoi told AFP.
"The most challenging threat would come from a lone-wolf attack. It is easier to monitor bad elements coming from identified violent groups." Still, Banlaoi said Philippine security forces had proved it could secure visits by popes and US presidents.