The Catholic Church has been haunted for decades by its failure to punish pedophile priests, but the crisis escalated last year, tarnishing Francis' papacy and raising the pressure on him to act.

After a string of scandals in Chile, the US and elsewhere, the pope admitted in August the pain of abuse victims "was long ignored, kept quiet or silenced."

A month later, the pope announced the summit.

Some 190 officials are invited, including 114 head bishops from national church organisations around the world, as well as 10 representatives of women's religious orders.

Before travelling to Rome, head bishops were urged to meet with clergy sex abuse victims in their countries for a firsthand account of their suffering.

The summit is organised like an academic conference, with presentations, working groups and plenary debates, plus prayers on each day.

Abuse survivors are also due to speak, as is a veteran Vatican affairs journalist, who will brief bishops on how to communicate with the public.

The first day is due to focus on the theme of responsibility; the second on accountability; the third on transparency. The summit wraps up Sunday with Mass and a closing speech by the pope.

There have recently been concerns about the sexual abuse of nuns at the hands of priests and bishops, but it is not on the agenda.

Conservatives are clamouring for a strong summit condemnation of homosexuality, even if the link between homosexuality and sexual abuse has been rejected by multiple studies.

Francis was the first to lower the bar of last month, when he said that: "expectations need to be deflated."

He explained that meeting could not put a stop to abuse, but would serve to "take consciousness" of the problem and adopt clear guidelines against it.

An overriding objective is to raise awareness of the abuse crisis in countries where local Catholic leaders have so far paid scant attention to it.

Father Federico Lombardi, a member of the organising committee, said task forces could be sent to countries where anti-abuse measures must be strengthened.

Phil Saviano, an abuse victim and campaigner who helped uncover the infamous "Spotlight" cover up scandal in Boston, US, is not holding out for any breakthrough.

"I don't expect them to be coming out with any new rules or regulations in four days," he said Wednesday, after he and other abuse survivors met with summit organisers.

The conference will mostly be "an educational session" for "bishops from parts of the world where they think they don't have this problem," Saviano predicted.

Saviano's Bishop Accountability and other advocacy groups for victims of predator priests have flocked to Rome to hold several protests.

Their demands include the mandatory expulsion from the clergy of any predator priest and bishop guilty of cover-up.

Another long-standing request is the introduction of a blanket obligation for bishops to report to police any suspect priest under their watch. Right now, they are only encouraged to do so.

Australian Associated Press