Many paused to share photographs and other mementos of children and young people who have died or are battling cancer because of the pollution.
Leo's visit to the so-called Terra dei Fuochi, or Land of Fires, came on the eve of the eleventh anniversary of the late Pope Francis' big ecological encyclical Laudato Si (Praised Be), and indicates Leo's commitment to carry on his predecessor's environmental agenda.
"I have come first of all to gather the tears of those who have lost loved ones, killed by environmental pollution caused by unscrupulous people and organisations who for too long were able to act with impunity," Leo said in remarks to family members and local clergy inside Acerra's cathedral.
The pontiff recalled that the area now dubbed the Land of Fires was once called "Campania felix," Latin for blessed or fruitful countryside, "capable of enchanting for its fertility, its produce and its culture, like a hymn to life".
"And yet — here is death, of the land and of men," the pope said.
The European Court of Human Rights last year validated a generation of residents' complaints that mafia dumping, burial and burning of toxic waste led to an increased rate of cancer and other ailments in the area of 90 municipalities around Caserta and Naples, encompassing a population of 2.9 million people.
The court found Italian authorities had known since 1988 about the toxic pollution, blamed on the Camorra crime syndicate that controls waste disposal, but failed to take necessary steps to protect the residents.
The binding ruling gave Italy two years to set up a database about the toxic waste and verified health risks associated with living there.
Bishop Antonio Di Donna estimated 150 young people had died in the city of some 58,000 over the past three decades, emphasising in his opening remarks that the number didn't take into account adults and victims in other municipalities.
He urged the pope to admonish those who continue to pollute, noting that the dumping of tons of toxic waste was reported a day earlier near Castera.
Di Donna said that Italian officials had identified dozens more human-caused contamination sites throughout the country, including the Venetian port of Marghera, and the leaching of PFAS forever chemicals into groundwater near Vicenza.
"We say to those brothers of ours ensnared in evil and seized by a mirage of fabulous earnings: Convert, change your ways, because what you are doing is not only a crime, it is a sin that cries out to God for vengeance," the bishop said.
The pope later greeted the mayors of the 90 communities impacted by the toxic dumping, and greeted thousands of people waving yellow flags and chanting "Papa Leone" along the route of his pope mobile and in a central piazza.
The victims include Maria Venturato, who died of cancer in 2016 at the age of 25.
Her father, Angelo, said he hopes to speak with the pope to explain their reality, "not for me … for the next generation".
"I'd like to give these young people a future, so I'm asking for the pope's help with this. That is, I'm making a strong appeal to him to go to those in power and say, 'Look, let's heal this land of fires,'" he said.
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