On the last day of a historic summit on sexual abuse in the Catholic Church, the Pope has denounced predatory clergy as 'tools of Satan'.

Pope Francis told senior bishops gathered at the Vatican that sexual abuse was akin to human sacrifice as he promised to wage an 'all-out battle' to rid the Catholic Church of the scourge.

'If in the Church there should emerge even a single case of abuse – which already in itself represents an atrocity – that case will be faced with the utmost seriousness,' he said.

'I am reminded of the cruel religious practice, once widespread in certain cultures, of sacrificing human beings – frequently children – in pagan rites.' 

The summit was convened by Pope Francis to address the sexual abuse scandals that have affected numerous countries, including Australia, the US, Germany, and Chile. 

The Pope Francis said the Church was standing 'face to face with the mystery of evil' and that children needed to be 'protected from ravenous wolves'. 

'I make a heartfelt appeal for an all-out battle against the abuse of minors both sexually and in other areas ... for we are dealing with abominable crimes that must be erased from the face of the earth,' he said. 

He made the comments after leading bishops in a penitential service in which they were told to examine their consciences and confess their sins. 

'We must say, like the prodigal son: 'Father, I have sinned',' he said.

But some victims were left disappointed by the summit. 

Swiss victim Jean-Marie Furbringer told AFP, 'Honestly it's a pastoral 'blabla', saying it's the fault of the devil.'

Another victim, Peter Saunders, from Britain, told the news agency he wanted more concrete action to be taken against abusive priests.

'It talks about the devil, it talks about evil... There is no talk about permanently excluding child rapists and abusers who are employees of the Church,' he said.

'"From a child protection point of view it's been a waste of time. From keeping the world's eyes on this institution and its failure to protect children, it's been a great success.'