Catholic leaders asked God for forgiveness for the crimes of paedophile priests at a church service today after an Irish victim said the Vatican should be held accountable for destroying lives.

The heads of seven religious orders intoned penitential prayers in a church in Rome on the sidelines of a first-ever Vatican summit to look at ways of rooting out child abuse from the Church after thousands of scandals.

"We have sinned. We did not know how to listen to the pain of so many innocent ones," one of the bishops taking part in the service said.

"We are aware that our acts of reparation can never erase the unjust things we have done or soothe the searing wound of our consciousness," he said.

Victim Marie Collins earlier told the conference at the Vatican's Gregorian University: "Apologising for the actions of the abusive priests is not enough.

"There must be acknowledgement and accountability for the harm and destruction that has been done to the life of victims and their families by the often deliberate cover up and mishandling of cases by their superiors."

Collins said her abuse happened at the hands of a hospital chaplain when she was just 13. "Those fingers that would abuse my body the night before were the next morning holding and offering me the sacred host," said the 64-year-old.

Her denunciations were long ignored by the Catholic hierarchy and she has suffered from depression for most of her life as a result, but Collins has since become a leading voice in Ireland pushing for justice for victims.

The four-day conference opened with a message from Pope Benedict XVI urging "profound renewal of the Church at every level" and calling for "a vigorous culture of effective safeguarding and victim support."

The meeting brings together 100 representatives of national bishops' conferences, the leaders of 33 religious orders and the Vatican's top anti-abuse prosecutor, Mgr Charles Scicluna, as well as lay experts on the issue.

There cannot be a distinction between the good of the Church and the protection of the young.- Mgr Charles Scicluna

Collins was the only abuse victim to have been invited and victims' groups have dismissed the conference as "a public relations exercise". She said she struggled with the decision to attend after years of conflict with the Church.

She and other conference participants will take part in a special penitential church service later on Tuesday in which the leaders of seven religious orders that had abusers in their midst will plead for forgiveness.

US archbishop Stephen Rossetti also addressed the conference on Tuesday, listing mistakes made by the Church in dealing with abuse, such as being manipulated by offenders and believing perpetrators could easily be cured.

"It is time to pro-actively and aggressively root out this evil from our society. You and I must begin this task by exorcising it from our own midst... Child molesters must know they have no safe sanctuary in the Church," he said.

The Catholic Church has been rocked over the past decade by thousands of abuse scandals in Europe and the United States, which have shown up systematic cover-ups of known abusers by the Catholic hierarchy going back decades.

The Vatican summit aims to encourage countries where many Church abuses remain hidden, such as in Africa, Asia and Latin America, to learn from other places like the United States where more stringent practices are enforced.

The Vatican has requested that national bishops' conferences submit strict guidelines by May on how they plan to root out child abuse but campaigners warn that this is not enough as there is no real power to enforce these rules.

"Policies are useless unless there is a will and commitment behind them," Rossetti told the conference, stressing that Catholic churches around the world had to be aware of the potential for child abuse in their midst.

"Any denial of the frequent sexual molestation of minors around the globe is an echo of the very denial perpetrators use to keep their evil hidden.

"When we fail to expose this evil to the light of day, it continues secretly to pollute the Church from within."

Mgr Scicluna said all church leaders must be clear that the protection of children is a paramount concern for the Church:

“There cannot be a distinction between the good of the Church and the protection of the young.”