Pope Francis opened a landmark Vatican conference on the sexual abuse of children by priests on Thursday by saying the Church would 'listen to the cry of the little ones seeking justice'.

Victims expected 'concrete and efficient measures' to address the abuse and scandal and not mere condemnations, Francis said in a short opening statement.

The 'evil' of sexual abuse of children by priests had to be transformed into a 'purification' of the Roman Catholic Church, he said.

After the pope spoke, Cardinal Luis Tagle of the Philippines, who broke into tears as he read a keynote speech, acknowledged that 'wounds have been inflicted by us, the bishops, on the victims'.

The four-day conference brings together the heads of national Catholic bishops conferences, Vatican officials, experts and heads of male and female religious orders.

Some victims' groups have voiced scepticism, calling the event a publicity stunt aimed at cleansing the image of the 1.3 billion-member Church which has been severely tainted by the scandal that is now more than three decades old.