Vatican treasurer Cardinal George Pell says the Catholic Church has made "enormous mistakes" in dealing with claims of sexual abuse.
He is giving evidence from Rome via video link to an Australian inquiry into child sex abuse.
Cardinal Pell, Australia's most senior Catholic, will be asked whether he knew if paedophiles were active in churches under his watch.
"I'm not here to defend the indefensible," said Cardinal Pell.
"The Church has made enormous mistakes and is working to remedy those but the Church in many places, certainly in Australia, has mucked things up, has let people down," he said.
Abuse survivors have flown to Rome to face the cardinal, who was excused from returning to Sydney due to ill health.
The hearing, which began this morning, is expected to run for at least three days.
Cardinal Pell is not himself accused of sexual abuse.
The Royal Commission into Institutional Responses to Child Sex Abuse is holding its second round of inquiries into child sex abuse that occurred in the city of Ballarat in Victoria state.
Cardinal Pell was a priest in Ballarat and lived in a seminary with a notorious paedophile priest, Gerald Ridsdale, in the early 1970s.
Ridsdale committed more than 130 offences against young boys while working as a chaplain at Ballarat's St Alipius school between the 1960s and the 1980s.
Even though Cardinal Pell is not facing criminal charges, detractors have questioned the extent of his knowledge of child abuse, and say it could make his Vatican position untenable.
Cardinal Pell is speaking before observers from a Rome hotel and proceedings will be streamed live to the Royal Commission's hearing rooms in Sydney and Ballarat Town Hall, and will also be viewable online.
'Time we saw action'
In May 2015, during the first round of inquiries into what happened Ballarat, the commission heard evidence that Cardinal Pell tried to bribe Ridsdale's nephew to stay silent about the abuse.
Cardinal Pell has strongly denied the allegation.
Anthony Foster, whose daughters were abused by a paedophile priest, will be in Rome as part of his search for answers.
"We want to hear the truth. [Pell has] worked his way right through the hierarchy, right up to the top of the Catholic Church. So we really want to hear the truth about what happened and it's about time we saw some action," he told the ABC before he left for Rome on Friday.
Cardinal Pell and the Royal Commission
- Cardinal Pell, Australia's most senior Catholic, was appointed inaugural prefect of Holy See finances in the wake of scandals at the Vatican Bank
- He was previously Archbishop of Sydney and before that Archbishop of Melbourne
- He held positions in and around the Victorian city of Ballarat in the 1970s and 1980s
- The Royal Commission into Institutional Responses to Child Sex Abuse was set up in 2012, largely in response to allegations surrounding the Catholic Church
- Cardinal Pell has testified before the commission twice already over matters not related to Ballarat
- The cardinal was excused from attending this round of the hearings due to a heart condition, but critics have accused him of avoiding victims
- A group of victims has flown to Rome to hear his testimony, which will be given from a hotel function room.