Pope Francis has launched a stunning attack on clerical abuse victims in Chile, accusing them of slandering a bishop they accuse of protecting a paedophile priest.

The comments caused outrage in South America in the midst of a complex and delicate tour where the pontiff has faced protests and threats of violence over his handling of abuse allegations.

Pope Francis told reporters there was no evidence against Bishop Juan Barros Madrid, who he appointed as Bishop of Osorno despite accusations he was complicit in hiding the crimes of Rev. Fernando Karadima, Chile's most notorious priest.

'The day someone brings me proof against Bishop Barros, then I will talk,' Francis said before celebrating Mass outside the northern Chilean city of Iquique. 'But there is not one single piece of evidence. It is all slander. Is that clear?'

The remarks caused fury among campaigners with Anne Barrett Doyle, a co-director of BishopAccountability.org, which monitors abuse cases, saying his 'attack on the Karadima victims is a stunning setback'.

She said: 'He has just turned back the clock to the darkest days of this crisis. Who knows how many victims now will decide to stay hidden, for fear they will not be believed?'

Karadima himself was convicted of abusing teenage boys by the Vatican in 2011 and sentenced to a 'life of prayer and penitence'. A criminal against him was dismissed because the statue of limitations had run out despite a judge finding the allegations to be 'truthful and reliable'.

Victims of Karadima allege that Bishop Barros knew about the abuse and was part of Karadima's inner circle.

However Francis has supported Barros and backed his promotion to Bishop of Osorno in 2015, saying there is no evidence against him.

It is not the first time the pontiff has provoked anger among Barros' opponents. In 2015 he told tourists visiting the Vatican the protestors were 'stupid and lefties'.

'The Osorno community is suffering because it's dumb,' he said, according to video recorded by one of the tourists. The city had 'let its head be filled with what politicians say, judging a bishop without any proof'.

But despite his staunch support for Barros, Pope Francis has gone to great lengths to apologise for the abuse carried out by Catholic priests in South America.

In his first speech of the week-long tour he begged for forgiveness from sex abuse victims and said he feels 'pain and shame' over the scandal, apologising for the 'irreparable harm' done.