Pope Francis has condemned the Mafia for exploiting and enslaving people, calling on gangsters to repent in words that recalled an impassioned plea 20 years ago by Pope John Paul II.

Speaking off the cuff after his weekly Angelus blessing in St Peter's Square yesterday, Francis spoke about the Mafia for the first time since he became pontiff two months ago.

High-profile killings by the Italian Mafia have declined since the 1990s, but through prostitution, extortion and drug trafficking it still wields a heavy influence over the country and its economy.

The UN says Italy's main crime groups – the Sicilian Cosa Nostra, the Calabrian 'Ndrangheta and the Camorra from the Naples area – have an estimated annual turnover between them of £100 billion, more than the annual sales of Italy's biggest firm, oil giant Eni.

Francis recalled the example of the Sicilian anti-Mafia priest Giuseppe Puglisi, who was killed by gunmen in 1993 outside his home in the island's capital of Palermo, and was beatified on Saturday.

"My thoughts are with the suffering of women, men and also children who are exploited by the many mafias who make them slaves, through prostitution, through many social pressures," he said.

"They cannot do this, they cannot make our brothers slaves, we must pray to the Lord to make these mafiosi convert to God."

In one of his most famous addresses, in Agrigento in Sicily in 1993, John Paul angrily called on mafiosi to "repent, because one day you will face the judgment of God".

An estimated 100,000 people attended the seafront ceremony on Saturday during which Father Puglisi was beatified.

He was shot in the back of the head in the crime-ridden Brancaccio district of Palermo where he preached against the Mafia.

Francis's predecessor, Pope Benedict, put Puglisi on the fast track towards sainthood in the Roman Catholic Church when he signed a decree last year recognising Puglisi had been killed "in hatred of the faith."

That meant he could be beatified – the last step before sainthood. A miracle is required for him to be declared a saint, however.

A huge photo of Puglisi was unveiled as white doves were released during the two-hour ceremony in a vast square surrounded by palm trees. He was repeatedly referred to as a martyr who paid for his constant preachings against the Sicilian mob.

He was killed for "depriving the Mafia of support, manpower and control of the city" with his large following in Palermo, said Cardinal Paolo Romeo, the primate of Sicily who presided over the ceremony.

Many of the faithful wore T-shirts bearing photos of Puglisi. Italian President Giorgio Napolitano said in a statement that Puglisi "continues to be an example for all those who refuse to give in to the power of organised crime."

Six mafiosi were jailed for life for his murder, which took place a year after the high-profile killings in Sicily of anti-Mafia prosecutors Giovanni Falcone and Paolo Borsellino.

The Cosa Nostra is probably the best known of Italy's main criminal organisations, but in the last 20 years many of its top bosses have been captured. The Camorra and the 'Ndrangeta are now considered more powerful.