Pope Francis used a pastoral visit to the southern Italian city of Naples to attack corruption and crime.

Known as a Mafia heartland, the city has suffered from decades of organised crime since the Second World War.

Speaking to a crowd of thousands in its Plebiscito Square, the Zenit news service reporting him saying: "To the criminals and all their accomplices, today, I, humbly, as a brother, repeat: convert to love and justice."

"Do not give in to the lure of easy money or dishonest income," he said. Those who are truly blessed, he added, "are those poor in spirit, the non violent, the meek, and the peacemakers."

He told Neapolitans that they should "React firmly to organisations that exploit and corrupt the young, the poor and the weak," especially those tied to crime and drugs, and called on them not to allow corruption and crime "disfigure the face of this beautiful city and the joy of the Neapolitans".

To criminals, he said: "With the grace of God, who forgives all and always forgives, you can return to an honest living."

On his arrival in the city he was transferred by helicopter to a sports field where he met crowds including unemployed people and immigrants. "I wanted to begin my visit at this periphery," he said, in a speech returning to his frequent theme of the Church being for the poor and marginalised.

Referring to Naples' problems with immigration and 40 per cent unemployment, the Pope asked: "Do I really have to tell you that immigrants are children of God? ... Are immigrants second class citizens? No! Immigrants are citizens like us."

He said that "Lack of jobs for young adults is a grave problem for society. Unemployment robs one of dignity." He also warned employers to treat staff fairly, referring to one young woman offered work in the tourism sector for €600 a month for 11-hour days. "This is called slavery, this is called exploitation," he said.

Francis also visited the Poggioreale prison, dining with some of the inmates in a large room normally used as a chapel. Italian media reported the inmates prepared a simple pasta meal and a group of transsexuals were among the group.

He also visited the city's cathedral, where the Archbishop of Naples, Cardinal Crescenzo Sepe, told the crowd there that the dried blood of St Gennaro, kept in an ampoule, had begun to liquify.

Appearing surprised, Francis took back the microphone and said: "The bishop said the blood is only half liquefied. It appears the saint only loves us half-way. We have to convert ourselves (to good) more so he loves us more," Francis said.

Sceptics say that the blood liquifies when chemicals in the ampoule react when it is moved; the Pope seems to have deftly avoided taking a position on the question.