Two Italian journalists who wrote books detailing Vatican mismanagement have gone on trial along with three people accused of leaking them the information.

Gianluigi Nuzzi and Emiliano Fittipaldi face up to eight years in prison if convicted of charges they violated Vatican law by publishing news based on confidential Holy See documents.

The Committee to Protect Journalists (CPJ), Reporters Without Borders and the Organisation for Security and Cooperation in Europe (OSCE) have all called on the Vatican to drop the charges against the pair.

Mr Nuzzi and Mr Fittipaldi both called the process "Kafka-esque".

With hours to go before the start of trial, neither they nor their lawyers had seen the court file detailing the accusations against them.

Mr Nuzzi only spoke with his Vatican court-appointed lawyer for the first time on Monday morning. They were indicted on Friday.

Even though they technically risk arrest by stepping on Vatican soil, both arrived shortly before the trial was due to begin, saying they want to understand the accusations and to report to the world what transpires.

"We are serene," Mr Nuzzi said, adding that the two would not be intimidated.

"There is evidently an interest in distracting from the embarrassing disclosures," he said.

Mr Fittipaldi said they did not yet understand what they were being accused of doing.

He added: "The only thing we did was publish the news, which was not denied, and documents that tell of scandals."

Neither expected the Vatican would detain them, given the diplomatic incident it would set off with Italy.

Since Mr Fittipaldi and Mr Nuzzi are Italian citizens, any sentence would presumably involve an extradition request.

Both journalists said they believed no Italian judge would extradite them, given freedom of the press is guaranteed by the Italian constitution and that Italy would be loath to extradite two of its citizens to a state that does not respect the same fundamental right to free expression.

Mr Fittipaldi's book Avarice, and Mr Nuzzi's book Merchants In The Temple, both published earlier this month, detail waste and mismanagement in the Vatican administration, the greed of some cardinals and bishops and the resistance Pope Francis is facing in trying to clean it up.

Much of both books was based on documents produced by a reform commission Francis appointed to get a handle on the Vatican's financial holdings and propose reforms so that more money could be given to the poor.

The three other people on trial are Monsignor Angelo Lucio Vallejo Balda, his assistant Nicola Maio, and Francesca Chaouqui, a commission member and public relations expert.

All three are accused of forming a criminal organisation and of procuring and leaking confidential documents.

Mr Nuzzi and Mr Fittipaldi are accused of publishing those documents and of "soliciting and exercising pressure, above all on Vallejo Balda, to obtain the documents and other reserved news", according to prosecutors.

The journalists deny the pressure accusation but acknowledge that they, like all journalists, obtained information and published it.

Ms Chaouqui has denied wrongdoing and was allowed to avoid detention after she co-operated with investigators.

Mr Balda, who is in Vatican detention, and Mr Maio have not responded publicly to the accusations.