In his most thorough admission of the Roman Catholic church's guilt in the clerical sex abuse scandal, Pope Benedict XVI has said that the greatest persecution "is born from the sins within the church", not from a campaign by outsiders.

The pontiff said on Tuesday that the church had always been tormented by problems of its own making, something being witnessed now "in a truly terrifying way".

"The church needs to profoundly relearn penitence, accept purification, learn forgiveness but also justice," he said. "Forgiveness cannot substitute justice," he said.

Benedict was responding to journalists' questions, submitted in advance, aboard the papal plane as he flew to Portugal for a four-day visit.

In a shift from the Vatican's initial claim that the church was the victim of a campaign by the media and abortion rights and pro-gay marriage groups, Benedict said: "The greatest persecution of the church doesn't come from enemies on the outside but is born from the sins within the church."

Previously, he had taken to task the abusers themselves and, in the case of Ireland, the bishops who failed to stop them.

Benedict has promised that the church would take action to protect children and make abusive priests face justice.

He has started cleaning house, accepting the resignations of a few bishops who either admitted they molested youngsters or covered up for priests who did.

Critics are demanding more. They recall that while Benedict has scolded his church and accepted some bishops' resignations, none of them has been actively punished or defrocked, even those who admitted molesting children.

David Clohessy, director of the main US victims' group, the Survivors Network of those Abused by Priests, said "many are tiring of hearing about his 'strong comments'. They want to see strong action".