Five people, including two Italian reporters, went on trial in the Vatican on Tuesday, to outrage from rights' groups, on charges arising from publication of books in which the Holy See was portrayed as mired in mismanagement and corruption.

As they walked into the Vatican, the two reporters, Gianluigi Nuzzi and Emiliano Fittipaldi, said they had done nothing wrong and were merely doing their professional duty.

The trial, being heard by three judges, stems from publication of two books which depict a Vatican plagued by mismanagement, greed and corruption and where Pope Francis faces stiff resistance from the old guard to his reform agenda.

Two of the officials indicted, Spanish Monsignor Angel Lucio Vallejo Balda, who was number two at the Vatican's Prefecture for Economic Affairs, and Italian laywoman Francesca Chaouqui, a public relations expert, were arrested earlier this month.

Balda and Chaouqui were both members of a non-defunct commission Francis set up in 2013 to study economic and administrative reforms. Vatican employee Nicola Maio, Balda's assistant, also went on trial.

The Holy See was embarrassed and angered by the books, which it said used information that should never have been allowed to leave the walls of the city state.

The Vatican has said "Merchants in the Temple" by Nuzzi and "Avarice" by Fittipaldi, give a "partial and tendentious" version of events and has accused the writers of trying to reap financial advantages from receiving stolen documents. The books were published earlier this month.

The leaks are one of the biggest internal scandals to hit the papacy of Pope Francis and are reminiscent of the "Vatileaks" furore that preceded the resignation of former Pope Benedict in 2013. The Italian media has dubbed the latest episodes "Vatileaks II".

In their indictment issued last Saturday, prosecutors said Balda, Chaouqui and Maio formed "an organised criminal association" with the aim of "divulging information and documents concerning the fundamental interests of the Holy See and the State".

Nuzzi and Fittipaldi, wrote books based on the leaks. Their indictment said both "solicited and applied pressure, especially on Vallejo Balda, to obtain secret documents and information.

Both have complained that they have had to accept court-ordered lawyers accredited to the Vatican and not allowed to use Italian lawyers.

The five defendants risk jail sentences of up to eight years.

On Monday, the human rights watchdog, the Organization for Security and Cooperation in Europe (OSCE), urged the Vatican to withdraw the charges.

"Journalists must be free to report on issues of public interests and to protect their confidential sources," the OSCE's representative on freedom of the media, Dunja Mijatovic, said.

"I call on the authorities not to proceed with the charges and protect journalists' rights in accordance with OSCE commitments," she said.

The Italian journalists' federation, Italy's foreign press Association and AIGAV, the association of reporters accredited to the Vatican also condemned the indictment of the journalists.

"We have to stress that publishing news is precisely our job," AIGAV said, calling the decision to try the journalists "unacceptable".