The pontiff arrived in the poor, crime-ridden Scampia area of the city in a popemobile and was immediately plunged into a crowd of children and young people, two of whom managed to pose for a selfie with the pontiff.
"Corruption stinks, corrupt society stinks," he told residents, adding that "we all have the potential to be corrupt and to slip into criminality".
Up to 800,000 people were expected to turn out throughout the day in the southern Italian city to greet the Argentine pontiff, who last year declared war on organised crime by "excommunicating" all mafiosi from the Catholic Church.
Security was tight for the visit: apart from risks posed by gangsters with a grudge, the pope has been threatened by the Islamic State group and trips outside the Vatican are considered prime opportunities for assassination attempts.
After a massacre at a Tunisian museum this week claimed by the IS group, all eyes with be on the elderly pontiff's safety, with 3,000 extra policemen deployed along the route he will take, including snipers on the rooftops.
Before his arrival in Scampia, Pope Francis began the day at the ancient Roman city of Pompeii.
He was later due to hold mass in the Piazza del Plebiscito in Naples' historic centre, a stone's throw from the Gulf of Naples, before visiting the city's overcrowded Poggio Reale jail, where 2,500 prisoners are squeezed into a space for 1,400.
Corruption, drugs, extortion
The pope, self-proclaimed defender of the downtrodden, will lunch with 90 of the inmates, including a dozen transsexuals, homosexuals and AIDS sufferers, according to the Catholic TV2000 television channel.
In Naples' Cathedral he will pray in front of the relics of the city's patron saint Januarius, where some of the former bishop's blood in a vial is said to miraculously liquefy three times a year, according to popular legend.
The visit will wind up with a sea-front concert of Neapolitan songs performed for the 78-year old pope by youngsters.
Francis is expected to speak about the ills weighing upon the region, from endemic corruption to drug addiction, sky-high unemployment and environmental pollution in an area scarred by the illegal burning of toxic rubbish.
His fiercest words will be kept for the Camorra, the notorious Naples mafia dissected and exposed in journalist Roberto Saviano's award-winning novel "Gomorrah" and the acclaimed 2008 film of the same name.
Last year the pope expelled all unrepentant mafia members from the Church and condemned them to Hell in the afterlife in a move which sparked fears of retaliation from the underworld.
City of 'many wounds'
During a visit to territory controlled by the powerful 'Ndrangheta organisation in the southern Italian Calabria region, he described the mafia as "the adoration of evil and contempt for common good."
"This evil must be beaten, expelled," he said, calling for historic ties between organised crime and the Church to be severed — an appeal he is likely to repeat in Naples.
Numerous priests fight against Italy's big three — the Cosa Nostra in Sicily, the 'Ndrangheta in Calabria and the Camorra in Naples – sometimes paying for their bravery with their lives.
But the Italian Church also has a darker side. Mafia dons have historically attended mass, often receiving lavish funerals and presenting themselves as Catholic benefactors, aiding local residents where the state has failed.
Priest Luigi Ciotti, who has dedicated his life to the fight against themafia, said the pope's visit would mean a huge amount to a city he described as "marvellous but bitter, generous, but living with many wounds."