A tide of lurid speculation, questionable accusations and possible blackmail attempts is buffeting the Vatican following Pope Francis's claim that he is preparing to tackle a gay lobby secretly at work behind the Holy See's walls.

The new pope's private comments to a group of visiting South American churchmen, which caused a sensation when they appeared on a religious website last week, prompted blushes in the Vatican and unleashed feverish gossip in Rome regarding the contents of a report on Vatican infighting prepared last year for Francis's predecessor, Joseph Ratzinger.

On his retirement in February, Ratzinger handed his Argentine successor the dossier, which reportedly describes a lobby of gay, senior churchmen inside the Vatican, running a network of patronage while fighting off blackmailers.

The pope's unguarded remarks, which appeared to confirm the speculation, have fuelled a new round of accusations, beginning with a convicted paedophile priest, Father Patrizio Poggi, who last week named nine fellow prelates in Rome as part of a secret band who used a police officer to supply them with eastern European rent boys.

By Friday Poggi was under arrest, accused of defamation, as investigators claimed he had invented the whole story to take revenge on fellow priests who stood by when he was found guilty of abusing children.

On Thursday the anti-paedophile campaigner Francesco Zanardi posted a taped conversation on his website in which an informant told him about alleged secret sex parties inside the Vatican involving 14-year-old male prostitutes and cardinals, as well as rented apartments in Rome where prelates kept a supply of young men.

The man, who claimed to be involved in organising the parties, said he wanted to expose the Holy See's own "bunga bunga" scene, offering to supply photos and videos taken by the prostitutes and telling Zanardi:: "I can give you a hand."

"He named two cardinals … and the details he supplied stood up," Zanardi said. He bleeped out the names on the online recording, but has supplied all details to prosecutors, who have opened an investigation.

"In another call, the man put me in touch with one of the boys, who had just turned 18 … I talked to him and believed his story. He was known as 'the blond' and was one cardinal's favourite, but when I urged him to go to the police, he just said: 'It's my living.'"

Zanardi, 43, is on a mission to expose abuse and hypocrisy in the church after he was sexually abused in Liguria between the ages of 11 and 16 by a priest, Nello Giraudo, who had already been moved from one parish because of abuse allegations. The priest was later given an 18-month jail sentence.

An Italian journalist, Carmelo Abbate, who was also contacted by the anonymous informant, said he had yet to be convinced his stories stood up.

"He seemed like a man who wanted to take revenge … but he didn't come up with the proof," said Abbate, whose 2011 book Sex and the Vatican lifted the lid on the gay clubs and saunas frequented by Rome-based priests, including one who has held mass inside the Vatican.

Abbate has since published an exposé of priestly paedophilia in Italy. "During my research into abuse I came across many blackmailers, and decided they were almost as bad as the abusers, since they are trying to make money out of victims," he said.