Pope Francis has visited one of the most violent and drug-infested neighbourhoods of Naples and urged residents not to let organised crime and corrupt politicians rob them of hope.

Francis, on a day-long trip to the southern Italian city, addressed a crowd in the notorious Scampia neighbourhood, a stronghold of clans of the Camorra, the Naples version of the Sicilian mafia.

He spoke in the shadow of a dilapidated sailboat-shaped housing project known as Le Vele, so dangerous that even police are sometimes afraid to enter, residents say.

The blighted area has often been the battleground of Camorra clans fighting each other for control of drug trafficking and extortion rackets.

"See to it that evil is not the last word. It [the last word] has to be hope," he told a crowd of several thousand people as he was surrounded by children who rushed the stage to sit on the floor at his feet.

"Those who voluntarily take the road of evil rob a piece of hope. They rob it from themselves and from everybody, from society, from so many honest and hard-working people, they rob it from the good name of the city and from its economy."

Addressing the Pope in the Naples neighbourhood where drugs are sold openly and youth unemployment is more than 40 per cent, the city's archbishop, Cardinal Crescenzio Sepe, spoke of the "many evils that afflict us, such as crime and the Camorra".

Francis listened to a Filipino immigrant woman and an unemployed Italian man tell of their difficulties and a magistrate speak of "juvenile delinquency, desperation and death" in Naples.

The pontiff, often departing from his prepared address, defended immigrants, saying they could not be considered "second-class human beings". He called for just wages for workers and railed against corruption in public life.

"How much corruption there is in the world. I hope you have the courage ... to clean up the city and clean up society so that there is no longer that stink of corruption," he said.

Francis performs 'half-miracle' in Naples

During Pope Francis' visit, the dried blood of Naples' patron saint Januarius half-liquefied during a ceremony when he held and kissed the relic.

Archbishop Sepe showed the glass vial to the congregation in the city's cathedral and declared: "The blood has half liquefied, which shows that Saint Januarius loves our Pope and Naples."

Pope Francis, known for his plain speaking, quipped that he and his fellow visitors to the city's cathedral had failed to win the saint's full affection.

"The bishop just announced that the blood liquefied. We can see the saint only half loves us."

"We must all spread the word, so that he loves us more."

Each year thousands of Roman Catholic faithful go to the three special services at Naples Cathedral where the dried blood of the fourth-century martyr is said to turn to liquid.

Some people say the blood even increases in mass during these displays, though the church does not officially recognise any of the relic's alleged transformations as a miracle.

The Roman bishop was decapitated during the persecution of Christians during the reign of the emperor Diocletian in 305 AD.

Reuters